I dont really know how to make your legs not jump up, chayba. When i get the big vibration it usually takes my breath away and makes my body feel really un easy. Nothing jumps, though. You can try to just let your legs jump up. If you do this everytime you fall asleep then it could just be normal routine for your body.

The "theory" is actually very easy to be proven by anyone who AP's... just look at someone when they are sleeping when you are in the real time zone and you will see them above there person.