The ZenSlap, in whatever form, shakes together preexisting knowledge in the recipient's head, so they see what they (in principle) already know but didn't see. The right slap can bring two thoughts together that had been sitting next to each other, but unconnected, for a long time... or a set of thoughts needing only one more to make a complete set. At the wrong time, a slap (finding only the one thought) only hurts. The art of the master is to recognize when the thoughts are there.