sleep paralisis, shadow people or what?

one of my friends was talking about how a couple of times when he was sleeping he would wake up and his legs would be being held up like a foot off the ground and could see something holding them, but once he would concentrate on what was holding them up it would vanish and his legs would drop back down to the bed. also he would be able to movie his arms and yell during this but not move his legs so what happend?