>>271994998
I'm sorry, that isn't quite what I meant and I did an awful job of explaining it. 
It's to do with the act of holding your hands and arms like I described and moving them in and out. When you suddenly attempt to squash the ball you actually fight against your own muscles just slightly. 
It is very similar, mechanically, to the "Stand in a doorway and push your arms against the sides for 10 seconds now step out, whee your arms are floating!" thing.