OK, so
1. Attempting to trip an opponent , but he manages to maintain his feet is play-on?
2. Attempting to kick a player on the ground and missing is play-on?
3. A player on the ground attempting to tackle an opponent is play-on?
4. A player attempting to kick the ball out of an opponents hands when the opponent is attempting to score a try is play-on
5. You're the referee, and a player takes a swing at you. That's play-on is it?
Too many people forget this bit of the Law Book
"SPIRIT
Rugby owes much of its appeal to the fact that
it is played both to the letter and within the Spirit
of the Laws. The responsibility for ensuring that this happens lies not with one individual - it
involves coaches, captains, players and referees."
IMO, trying to knock the ball into touch, whether directly or off an opponent, to prevent a try being scored (even if the attempt fails) is definitely against the spirit of the Laws of the Game.