Thank you Coach.
One question rised up while reading this. The shortened ROM can not be the only advantage raising your butt brings. In limit reps, my butt will come up (or at least want to come up*) at the sticking point, when the bar is alread several inches above my chest, almost halfway up. That means, the ROM will not be shortened at that point, it's only the top half of the ascent left to do and raising the butt definitely helps.
So, why is that? Lat recruitment would be my guess?
* the butt wants what the butt wants.