I thought they explained that pretty well - the kid went limp and refused to get off the ground. Ever try to hoist a 175-pound limp sack? Imagine an inert dummy, and that's exactly how I'd expect to see someone pick it up - a lift and hoist to gain momentum. Throw in a little more vigor perhaps than was required because of the heat of the moment and he bounces. It doesn't take much to bounce up off a trunk lid either.