Consider the following scenario:
$1-2 NL cash game. You're in the big blind with:

Under the Gun calls the $2. Bears Fan is in middle position and raises to $10, clearing the field until it gets to you. You call; UTG folds. there is $25 in the pot. Flop hits:
You know Bears Fan has the better hand but you like that flush draw. You check. He bets $15, which makes you pretty sure he hit high pair. You put him on AxTx. The odds aren't in your favor but you call nonetheless. Fourth street gets you nowhere:
This presents a bit of a problem. If you check, you'll be broadcasting your draw. You want to see the river but certainly don't want to pay a lot for it. You've invested $25 into a $55 pot and have 11 outs (9 diamonds + 2 Queens). You're out of position. He's going to pound on you. What do you do?
One technique is to shoot out a "blocking bet," which the folks over at pokerhelper.com define as "... a bet made by an out-of-position player with the
intention of preventing an opponent in position from making a
problematic bet when checked to."
The benefits of the blocking bet is that it confuses your opponent by seizing control of the betting. It changes the reads people have on you. And when done correctly, it slows down the pot and allows you to see the next card at a price that's right for you.
For a blocking bet to work, it needs to be high enough that your opponent won't go over the top on it but low enough to make good financial sense to you.
For this pot, I assume my check would encourage Bears Fan to bet $30, so I throw in a blocking bet of $20. The move knocks him back, and he wonders aloud if I hit my jack or if I had two-pair to begin with. Dumbfounded, he decides the call isn't worth it and lays down his pair.
Of course, this is the best case scenario. The anticipated result would be that he would simply called the bet and we'd see the river. Nonetheless, the strategy worked, and it's something to consider next time you get too far down the path with a flush or straight draw.
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