Where I am from, tapping the thigh/butt is a sign to riders behind to slide over due to some above-ground obstruction. Often that obstruction is a (double) parked car, but it can be something like a runner proceeding in the same direction. Since we ride on the right side of the road, the tapping is of the right thigh, and it's not generally used to warn of things like potholes or glass. That's what the point is for.