Martial D
Senior Master
The main point is you want to put your hands closer to your opponent's head (this is why you use stiff arms) than his hands from your leg/legs. When he shoots toward your leg/legs, since the distance between his hands to your leg/legs is farther away from the distance between your hands to his head, your hands will be able to reach to his head faster than his hands can reach to your leg/legs (assume you and your opponent have the same speed).
In wrestling, it's called to lead your opponent into the emptiness (or let him to kiss the dirt). When you have body weight on top of his neck, it's not a good place for him to be.
I guess. I can't imagine any scenario where you get your rhino thing around someone's tucked head as they are crashing in for a shot, but you do you man.