…I had read somewhere that tapping on the can dislodged the micro bubbles that are clinging to the sides of the can allowing them to get back into solution more quickly than they otherwise might.
Looking for backup to this I found this link which describes this effect. It also mentions that it is difficult to know how successful you are prior to opening the can thus, as Cecil and others have mentioned, the best course of action is to let the can sit awhile before opening.