GeminiXcastorm

I know that universal does something with the launchers .json, but im not using the main launcher, so im curious as to what it does? and how i can get it to run properly? because the modded jar thats created after installing forge just runs vanilla when i use my custom launcher. I know that things like lite loader has some form of code that is run in the launcher right when the game is launched so it starts up correctly, what is it that FML does?