He wants his action to appear suspicious enough to lure a call.
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This is one of those rare tells that works better against strong players than against weak players. Sometimes, particularly in draw poker or seven stud, you'll meet an opponent who looks at the card or cards she's just caught (here, on seventh street) and prepares to bet at the same time. Many players feel bewildered and usually call the bet with weak hands. Wrong decision. When a player prepares to bet as she looks at her card, she's usually aware that her action will cause suspicion. In fact, she's using the tactic expressly to encourage a call. Because of that, she'll usually abandon this bet if she doesn't make a winning hand. Also, there's a good chance she already has the hand made and is looking and betting at the same time just to lure a call.
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Anytime you see a player seem to look at her card while beginning to bet, then complete her bet without hesitation, she's trying to fool you into thinking she's bluffing. You need a very strong hand to call this player's bet.
