I'm not sure about the implementation details, but I think the general logic would be as follows:

1) They sign a timelocked transaction using their private key, sending the coins to the target address but not redeemable until date x.
2) The timelocked transaction is stored on your server
3) Before date x arrives, they move their coins to a new address and sign another timelocked transaction using the private key of the new address.
4) Rinse and repeat until date x arrives when your server publishes the timelocked transaction to the network.

This way their private keys never touch the server, they can spent their coins however they like and the owner of the receiving address can't spend the coins until the dead man's switch has triggered.

Alternatively they could also lock a hardware wallet away in a bank tresor and have a dead man's switch email send the passphrase and PIN to unlock said hardware wallet in case of their demise as it's rather unlikely that someone would manage to prematurely get access to both.

So I am trying to write instructions for family members in case of death or whatever may be the case. I am in a situation where I have the regular 24 word seed phrase, but in addition to that I also use a pass phrase where the bulk of my bitcoin is stored. The idea is that if my 24 word seed phrase gets compromised somehow, the majority of my bitcoin are still safe until I can transfer it to an uncompromising wallet. Anyways, this additional complexity also complicates leaving instructions for family members who are not crypto savvy, and may not be literate in the world of technology. In my scenario, I use a ledger nano s hardware wallet with a temporary pass phrase. Ledgers website for instructions to set up a hardware wallet with a 24 word seed phrase are OK at best, so I wrote my own instructions. I would love advice on how to improve these instructions. I am trying to account for the fact that the person reading them may not have any idea how to do anything at all, but may be someone who is trusted with custody of my keys in case something bad happens (AKA: assume nothing, document in extreme and explicit detail). Additionally, I would love to expand this idea further in the future if these instructions are helpful as a template to write instructions for as many hardware wallets as possible, with as many configurations of possible recovery scenarios as possible. Anyways, here are the instructions I have written up so far.

What you'll need to gain custody of bitcoin on a ledger hardware wallet:

• Ledger Nano S with a  micro-USB cable (must be data and charging cable or device will not detect on computer)
• The 24 word recovery phrase to restore.
• An additional two-factor passphrase
• The Ledger Live application downloaded and installed. (instructions provided below)

Restore from a Recovery Phrase

• Once the device is plugged in, you will be greeted with some welcome text. Press both buttons at the same time to proceed. You may see more text that will require you to press both buttons again.
• When you get to the prompt to “configure as a new device” press the left button above “X”. This will lead you to the prompt asking if you want to “Restore Configuration” Press the right button above the check mark.
• The device will ask you to set up a PIN code. This code is requested every time you plug the device in. Press the left or right buttons to scroll through the numbers and press both buttons at the same time to enter a number. Do not forget this code or you will not be able to access the device. This code is not the same as a 24 word recovery phrase or passphrase.
• Once you have confirmed your pin code, you can enter in the 24 word recovery phrase to gain access and ownership of your Bitcoin.  You may need to press both buttons at the same time until you get to the prompt asking how many words you want to restore. Select 24 and press both buttons at the same time.
• You will see a prompt requesting you to enter the first word. Press both buttons at the same time to proceed. You will then see a prompt asking you to input the word letter by letter. Typically you only need to input the first two or three letters of the word and then it will give you a list of words to select. As an example, if the first word is “exchange” you will use the left and right buttons to find the letters “E” “X” and “C”. Once you have found a letter, press both buttons to input the letter.
• Once you have entered the third letter the device will try to help you fill out the rest of the letters by asking you which word you’re looking to input. Use the left and right arrows until you find the word you are looking for and press both buttons to confirm the word.  The device will then ask you to enter word two. Press both buttons to continue and repeat this process for all 24 words. Please note that if you input the wrong word at any point, you will have to unplug your device and restart this recovery process. Take your time. When you enter all 24 words successfully, you will see a prompt telling you your device has been set up successfully. Press both buttons to continue.

Setting up Ledger Live and enabling Bitcoin on your device. (PC)

• Go to https://shop.ledger.com/pages/ledger-live and download the software. Install the software to your computer and run it. This software will allow you to interact with your ledger device to send Bitcoin to and from the device. At first your device will not have the ability to manage Bitcoin. Ledger live will allow you to enable Bitcoin and set up an “Account” to see transactions.
• Once you have the software running, press “Get Started” and press “use an initialized device”.  On the “Select your device” screen, select Ledger Nano S
• You may see a “Security checklist” prompt. Follow the on-screen instructions where applicable. The software may check to ensure your device is an authentic ledger device.
• Press continue and you will be prompted to enter a password lock. This is only important if you don’t want other people on your computer to see your funds, but that is all it prevents. No one can spend your funds even with this password. Press “Skip this step” if you do not want to use a password.
• Press continue when you reach the “Bugs and analytics” page. Once you get to the “Your device is ready” page, press “open ledger live.”
• You should see a screen that says “instal apps or add accounts.” From here, press “Open Manager”. The software may prompt you to connect your device. Enter your pin code on the device to unlock it.
• When you see the prompt to “Allow ledger manager” on your device, click the right button above the check mark to allow the software to interact with your device.
• On the software you will immediately see a list of different cryptocurrency “apps” that you can install to the device. Find “Bitcoin” and press “Install”. You should see an “installing Bitcoin” popup and your device should read “processing”
• Once done, you will have “Bitcoin” and “Settings” to select from on your device. On the software, press “Add Bitcoin account” .
• The “Add accounts” popup should display. Chose “Bitcoin (BTC)” and make sure you get to the step where it asks you to “Open the Bitcoin app on your device”. DO NOT CONTINUE YET. At this moment, we will need to add the correct Bitcoin account. If you continue you will add the account that is not hidden by the two-factor passphrase. This may show a balance for some of your bitcoin, but not all of it.
• On your device, use the left and right buttons to navigate and select “Settings”. Press both buttons at the same time to open “Settings”
• You should see a list of options such as “Display” “Security” “Device” etc. Choose “Security” and you should see a new list of options. Choose “Passphrase”.
• In the passphrase menu, select “Set temporary” and then press the right button above the checkmark to confirm you want to enable the passphrase. When you see “Enter a secret passphrase” press both buttons to continue.
• The password may contain mixed numbers and letters, if it is a lower case letter you will need to select “ab” and enter in the letter. If its upper case, then “AB” and if its a number or special character, “?0”.  This step is similar to entering in your 24 words but a little more tedious. For example, if you have a passphrase like Fb43A then you will need to:
  
  • Navigate and select “AB” for capital letters.
  • Navigate and select “F”
  • Navigate and select “aA” to get back to the character type menu
  • Navigate and select “ab” for lower case letters
  • Navigate and select “b”
  • Navigate and select “aA” to get back to the character type menu
  • Navigate and select “?0” for numbers and special characters
  • Navigate and select the numbers “4” and 3”
  • Navigate and select “aA” to get back to the character type menu.
  • Navigate and select “AB” for capital letters.
  • Navigate and select “A”
• Once you have entered the last character of your passphrase, which will be different from the above example, navigate to the checkmark icon and select it to continue
• You will see “Confirm your passphrase”. Press both buttons to continue and you should see the passphrase you have entered. Ensure this is 100% correct. If it is wrong and you continue, you risk opening up an account with no bitcoin in it. Once you have confirmed its correct, press the right button above the check mark and enter your pin number when prompted.
• When you enter your pin number you should return to the menu prompting you to set a temporary passphrase. Scroll down until you get to the “Back” icon and press both buttons to continue. Scroll down again and press “back” to exit the security menu.
• You should see the “Security” menu option. Scroll all the way down until you get to “Quit app” and press both buttons to continue.
• Your passphrase will be enabled as long as the device is on. With it enabled and assuming the correct passphrase is used, you will now be able to add the correct bitcoin account in your ledger software on your computer and see your funds. On your device, go ahead and open the bitcoin app.
• On the software, click “Continue” and the “Add accounts” popup should appear and continue onto step 3. By default you should see “Bitcoin 1 (native segwit) selected”. Press Add account and then click “close” after you see the prompt “Account successfully added”. On the software you should be able to see your BTC balance, portfolio, and different navigation options to send or receive bitcoin. You do not need your device plugged in to see your balance, but you do need it to spend or receive any bitcoin. You will need to repeat steps 11 through 19 if you want to send or receive any bitcoin that is hidden using your passphrase.