I tap (about three times) on the tamping mat and still get some compression. I do this mainly to keep the mess in the one area so I dont get grief from the one who must be obeyed [smiley=embarassed.gif].
If I dont tap then the tamp goes deeper so there is some compacting of the grinds. I think there needs to be some experimentation on this by one of the science snobs to get a definitive answer.
I do (even with my limited understanding of physics) however agree that tapping on a solid object would give more effect.