The bosu thing was the right idea except you forgot how to squat all of the sudden. You kinda turned it into a hack squat...a version of which can be done like that with dumbells.
You are simply "folding up" like your body is a set of hinges or a piece of paper. You need to be focusing on opening the hips. Squat down between your knees. Think of it as like a yoga pose. You are practically falling into the wall. You still want to be squatting, only getting some help from the ball to add to stability..not just lying on it.
Look at this overhead squat:
http://i61.photobucket.com/albums/h6...ttom_248px.jpg