By appearing interested, the player is trying to tell her opponent that a bet is not safe.
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It's clear that players who stare at you are generally weaker than those who stare away. While this is a fairly powerful tell, it has its problems. Opponents may also watch your bet just to make sure you wager the right amount, so they won't get short-changed. You can sometimes be fooled into thinking an opponent is trying to act strong when, in fact, she's only monitoring the size of your bet. This tell poses that dilemma. If the player (center) was looking directly at the bettor's face, we'd have no problem. In such a case, it would be fairly clear that the bet was unwelcome. Even in this clip, however, you can usually assume that the opponent watching the bet is not strong.
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You can bet medium-strength hands into the player at center with the hope of getting a call from a weaker hand. Also, if you hold a hopeless hand, you might try bluffing.
