For some people their malocclusion causes stress on their TM joint - putting pressure where it shouldn't be. Either their bite is forcing their jaw too far back, forward, to the side, somewhere it doesn't want to be.
Now, if the problem is that your bite is forcing your jaw back too far because your front teeth are too close in, like me, then pulling the maxilla forward can correct your bite allowing your jaw to close where it's relaxed and meant to be. Then the TMJ may go away.
There may be other reasons that I'm not aware of, but that's at least one way "facepulling" can help tmj.