On the site it explains your passphrase is hashed using SHA256, and the output is used as the entropy needed to create a BIP39 seed phrase.

Interestingly, if you look at the source code, since it is mostly lifted directly from https://iancoleman.io/bip39/, it copied across the feature to perform a SHA256 hash if you are not using the "raw entropy" option, so in reality, your passphrase is hashed twice.

https://github.com/armorybrainwallet/brain2bip/blob/master/js/index.js#L292
Here is the first section of code where it SHA256 hashes your brain passphrase to generate your entropy

https://github.com/armorybrainwallet/brain2bip/blob/master/js/index.js#L1132
And then the second section of code where it SHA256 hashes your entropy a second time

You can test all this yourself. For example:

Inputting the string "thisisatest" as a Brain Passphrase, generates the following 24 word seed phrase: couch machine virus lion good camp topic maid common plunge history love where online case chest library shuffle obvious post okay force envelope birth

If you perform a SHA256 hash on "thisisatest", you generate the following 256 bit output: a7c96262c21db9a06fd49e307d694fd95f624569f9b35bb3ffacd880440f9787
If you perform a SHA256 hash on the above number, you generate the following: 30f0b3d241164641b944302e74ddb0c23fa535c8c93c8118ee62d4599eb612f0
If you then convert that second number in to binary, you get the following (which I've split in to groupings of 11):


You can then take each grouping of 11, convert it back in to decimal, and map it the relevant BIP39 word. 00110000111 in decimal is 391, which maps to "couch" at position 392 on the BIP39 word list (remembering to add one since your converted numbers will start at 0, whereas the word list starts at 1). 10000101100 in decimal is 1068, which maps to "machine" at position 1069. And so on.