I experienced this when I tried it for the very first time. I lied on my back like most people said, and didn't move a muscle. This is when I started to feel these minor waves that you might be talking about. Same thing happened to me with those small waves/vibrations. I would feel them and get excited and my heart would beat faster, then it would stop. I gave up on that and read some other stuff that said to just sleep in any comfortable position.

This is when I first felt SP. Trust me, you'll know if it's happening.

I suggest that you try to fall asleep normally how you usually do. You might end up falling a sleep for a brief moment but then wake up for a few seconds and feel SP kicking in. This is what happened with me, at least.