So the lead dev has a private key which lets him control all the MasterNodes? Why does he need to control all MasterNodes? In the future, will he be in control or is this just temporary and if so how long will this continue?
No, he does not. There is a private key for engaging the spork but that's it. This allows us to prevent forks for the time being while bugs are getting troubleshooted on mainnet when new features get rolled out.
If he chose to for whatever reason, could he crash/stop/impede the DRK network with this "masterkey"? What exactly does this key allow him to do?
He is refering to the Spork, is a message that is send out to the network so that you can activate, deactivate a hard fork. It works similar to the way alerts work in BTC. Is probably one of the coolest things on DASH/DRK and he chooses to troll about it. It poses absolutely no risk and gives no power, it just allows a mature network to innovate and do hard fork after hard fork in the name of progress, like DRK has been doing to implement its features.
From the OP:
Multi-Phased Fork Implementation (aka the Spork)
In response to unforeseen issues with the rollout of RC3, the Darkcoin development team created a mechanism by which updated code is released to the network, but not immediately made active (or enforced). Communication is sent out to users informing them of the change and the need for them to update their clients. Those who update their clients run the new code, but in the event of errors occurring with that new code, the clients blocks are not rejected by the network and unintended forks are avoided. Data about the error can then be collected and forwarded to the development team.
Once the development team is satisfied with the new codes stability in the mainnet environment - and once acceptable network consensus is attained - enforcement of the updated code can be activated remotely. Should problems arise, the code can be deactivated in the same manner, without the need for a network-wide rollback or client update.
This innovation allows for far smoother transitions than in the traditional hard fork paradigm, as well as the collection of test data in the live network environment.
We set out with the intention of calling this method of updating the Soft Fork, but the Darkcoin community quickly dubbed it the Spork and the name seems to have stuck.