What The tapping on yourself does, is give the horse warning for when (or if) you tap his withers. It works with my horse. Sometimes he won't go, so I tap my shoulers. He usually hears that and starts to go.
What it does, it it give the horse warnings for in case you have to actually tap his withers. Taping his withers can sometimes help a horse to go forward. They eventually will not like it,a nd try to get away from it - moving forward. The shoulder, waist and thigh taps are warnings. My horse goes as soon as he hears me tap my shoulders - I think it's because he doesn't want to get tapped, but I don't know for sure how it all works. But it does work.
It is a Parelli Technique, by the way.
Does that help? I don't really know any other way to explain it. But it really does work. When spring comes I'll try to post a video of my doing that.