He actually taps the head-but in front of the tension rod. Hes listening to the TYMPANIC tone. He goes from rod to rod and adjusts accordingly so the each match each other. Essentially, he uses his ear and listens-in effect, his ear is working just like a drum dial works. He really isnt doing anything magically-hes just listening to the tympanic tone at each rod location. Which in simple terms, hes just tuning really-
Try doing this tapping the rim just for the fun of it....bet you'll be surprised.