Milk Coffee Shake and Hit

So I have probably a stupid question, but nevertheless may be some of you might help me understand. Every time I get my capuccino in the morning, the guy that makes it takes the cup of milk, shakes it, then hits on the table a few times, then does the same thing for a couple of times, only after that pours into the cup.

Why does he do that? Why shake, then hit a few times, then shake again. I am not sure this is the right place to ask, but googling did not help me much. :)

Thank You, Eugene.