I've felt a variety of things. Sometimes it has felt like falling. Sometimes it has felt like I was 'shrinking' inside myself. Sometimes I get the feeling that I am hearing/feeling an intense lowish frequency vibration that gets 'louder' until it crescendos and then abruptly stops. Once it stops I'm in a total black silence and I know the transition is complete. Sometimes though, I feel nothing at all and these are obviously the hardest to recognize that I am actually dreaming.