You need at least 2 channels, one with a click track and the other with the backing track(s) (unless it's just the intro, or a one-time special effect). You feed the click into the drummer's ears and the backing track to the PA mains. The drummer must play to the click and everyone else follows the drummer to keep it all together.I'm curious since I've never used this sort of thing - is there any latency when triggering via a tablet? Are you pressing the button in time or is there some way of the software tracking your tempo and it starts it in time?