Hard to explain exactly what i feel, but i will try as good as i can.
First i get this heavy feeling in my body, which nearly feels like i sink a bit into my bed. Then i will get this strongly tingling feeling, which kinda feels like mild current"that i assume is what people call vibrations" going through your body, with the exception of lack of pain. I also in most cases hear a sound that sounds very much like wind blowing in your ears.
I sometimes also get a very vivid sensation of spinning or falling, but those feelings does not come in very beginning stage of my SP.
The only part of my body that i can willingly move, is the eyes.
I can however break out of SP, by concentrating all my strength on just moving for example my hand.
Once my hand starts moving, then the rest of my body will follow and i will now be fully awake.