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The second limper had been playing pretty loose.

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This is your reason for folding instead of pushing. You make moves like this against tight players that aren't calling you down as most of your equity comes from them folding. When you think you will be called more frequently you make this move less often. A9 isn't the best hand for this play as anyone with a bigger ace has you dominated and you're only slightly ahead of hands like KT-KQ, QT-QJ, JT. I prefer making this move with hands that aren't easily dominated like this one. Sklansky group 1/2 hands and small suited connectors (that take it all down when rags or a strong draw flop) are better than a medium strength hand like A9 here.