The player is unconsciously preparing to get involved.
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It's a mistake to think a player who suddenly adjusts his posture is acting in a manner he hopes will mislead you. Much more frequently, the player is unaware of his action, particularly if he's a novice. Note the times when players adjust their postures; you'll discover that it's usually when they are not conscious that you're watching them, making it unlikely that they're acting. Furthermore, the adjustment will usually be brief -- also indicating the player isn't trying to draw attention to himself. We don't know the exact strength of this player's hand, but we do know that he likes his cards.
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Unless this man holds a strong speculative hand, such as a king, queen or jack of the same suit, he probably has at least a pair of jacks. It's likely that he has a buried pair even higher than jacks or even three of a kind. So plan your tactics accordingly, enter the pot with caution and don't try to bluff.