SP is "sleep paralysis", and it's exactly what it says. One's body will become completely paralyzed; you won't be able to move. Some people seem to confuse being in a VERY relaxed state for SP. Make no mistake, in SP you are really paralyzed!

It's often accompanied by sensations of tingling and rumbling, and strange noises. Many people complain of a pressure on their chest, and difficulty breathing. SP can be scary, but with practice, this fear can be overcome. It is induced by staying conscious while one's body falls asleep. Being completely relaxed but mentally awake seems to be the key.

I hope that helped! Welcome to DV, btw.