I asked the Bone Man of NextShooter.com about this one. He said there are two reasons for the tap. The first is a way of thanking the player for the tip. A good dealer will verbally thank the player making the tip while making the tap. The second is a measure to keep the dealers honest. It has happened many times where dealers pocketed tips in casinos where tips are supposed to be put in a pool. Game security is not my strong point, but procedures like this are put there to make it harder to cheat, and easier to get caught if you do.