More recently, I've had more success with delaying the extension of the shoulders when reaching into the back handspring (the "arm throw" as ACoach describes). I think kids sometimes reach their arms back so early that their shoulders actually snap their arms back down toward their torso as their hands contact the floor. They also try to get their hands up so early that their ribs pop out and their knees actually move forward ("undercut").
During the late 90's, there was a trend where kids were taught to either not swing their arms or use a minimal arm swing, so the kids were effectively just jumping backwards onto outstretched arms, which, in my opinion, is very difficult and dangerous...and just biomechanically weird. I never understood that.
But the problem is not at all unusual, and even the best kids go through that phase at some point. All is not lost.