That's a difficult question and varies a lot...

When I enter a scene it's usually a sudden transition. For example, the transition in DEILD, and other wilding techs with visualization (VILD, etc) is very sudden for me - it like one moment you just imagine the scene, and suddenly it becomes REAL. Or when doing "general WILD" with pre-dreams present, the pre-dreams feel like a movie or something, until one of them feels 100% real (that's the dream that I stick with).

But when it comes to "leaving" your body - sometimes the dreams are still "not ready" - so it's like floating around without a body. This transition is pretty gradual for me... Usually I lose "sensations" part by part - from the feet up. It's not that things like temperature don't change - I just stop being aware of them. Like going blind / numb / deaf... Other times, instead of losing sensations, weird sensations gradually intensify - vibrations, floating, sinking, jolts, spinning, etc. They usually disappear relatively abruptly, though.