I have learned that we leave our body once it is asleep and astral project. This is because our soul doesn't sleep, only our bodies require this. It would be terribly dull to hang out with a sleeping body so we go to the astral plane. Most of us are not in a state of awareness while we are there, but people who lucid dream are aware while they are in the lucid state.

The rushing, falling feeling that you described so well is our soul/awareness re-entering the body.