The player wants to make you think he's unsure of his bet.
~
Hey, what can I tell you about a shrug that you don't already know? If you ask a friend how he's feeling and you get a polite shrug in response, that's supposed to substitute for, "I'm not really sure. Things could be better." This is exactly what a poker player is trying to convey when he shrugs and bets. Well, don't believe him! This player holds an almost certain winner.
~
Call only with powerful hands; in fact, just call with most hands that suggest a raise. Don't try to bluff.