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.