@Dab its happens when your mind wakes up before your body. Its that transition between sleep and awake. People tend to experience a black figure hovering around them or something holding them down on the chest. They are quite scary.
It's one of the worst things ever; i wouldn't even curse my enemies with it. once i had one so bad; as i was trying to twitch out of it; i twitched too hard and put my foot thru the wall.