A zen slap represents a moment of unexpected enlightenment when something becomes blindingly and undeniably obvious to you. It often brings a feeling of elation, relief, and discovery, even as you laugh at yourself for missing the obviousness all along.
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.
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See and add to Creativity (methods which could induce such Aha!!! moments).