Semi-Bluffs

A semi-bluff is a bet when you can win two ways, either because they fold or because they call and you make a draw to win.  Now, whenever you bet you have two ways to win the pot, either by them folding, or by them calling and you winning a show-down.  In the case of the semi-bluff we assume that you are betting big enough to put pressure on them, which means if they call you are beat and must make a draw.  Note that you cannot semi-bluff on the river, since you have no draw, and you cannot semi-bluff if you don't have a decent draw, you're basically just bluffing in those cases.

The classic case for a semi-bluff is when you have a flush or straight draw on the flop.  If you could see both of the next two cards, you have a 30-35% chance of winding up the winner, and the nice thing is you make a pretty big hand (a straight or flush), so you will beat most hands that call you if you hit your draw.  So, 30-35% is not good enough to call a pot-size bet (we'll ignore complicated considerations like the implied odds of taking his whole stack in No Limit Hold'Em).  However, if he checks and you bet a large amount, he may also fold.  If he does call, you have to hit your draw, but since you've shown strength he may likely check to you on the turn and let you see the river for free.  So, by betting you have increased your chance of winning over 50% - partly from his chance of folding, and partly from your chance of hitting the draw.  The best situation for you is to go all-in, assuming your stack is not too much bigger than the pot.  That puts a lot of pressure on him to fold, and if he does call you still have your 1/3 chance of winning.  Let's say you have 50 chips left and there are 30 chips in the pot.  He will fold 50% of the time, and then you win 1/3 of the remaining time.  So your EV is : 0.5 * 30 + 0.5 * ( 1/3 * (30 + 50) + 2/3 * (-50) ) = 11.666 chips.  The nice thing about getting all-in is that he can't charge you any more on the turn, so you know you are using all your chips to put pressure on him, not calling.

Now, if he someone knows you're bluffing or semi-bluffing, this becomes a bad play.  It's critical that he will fold a reasonable amount of the time; the exact amount of folding required depends on the pot size and bet size, but if he believes you actually have a hand (eg. top pair) so he will only be calling with top pair or better, then he will certainly be folding enough.  Note that in some cases he will also be on the draw, which is why you always want to be drawing to the nuts or nearly the nuts; if he has the same draw but better (eg. higher flush cards), you are drawing dead.

Another example of a semi-bluff if when you raise short-handed preflop with poor cards, or when you raise from the button after many folds.  In this case you can win if they fold, or if the flop comes in your favor, or just by them showing weakness and you bluffing again.  If you are a strong player, you can win the pot here any time they don't hit something big.  You don't actually have to hit your cards, you just need them to miss theirs, which happens a lot more often.  The reason you are considered to have a "decent draw" here is because any two cards are not that big of a dog pre-flop.

Consider the case that you are playing 3 handed.  Your stack has gotten low and you have only 3 times the big blind.  You are on the button, so you will be the big blind on the next hand.  It's a reasonable play here to go all-in with any two cards!  If you don't, you will be basically forced to go all-in on the next hand because of the big blind.  If you go all-in now, you can win two ways - either because they fold, or because you win the show-down.  Assuming the big blind will fold a reasonable amount here, or even that he folds a bit too often (which is a very common mistake), you maybe have a 50% chance of winning the blinds, and then a 1/3 chance of winning if he calls.  So your overall EV is 0.5 * 1.5 + 0.5 * ( 1/3 * ( 1.5 + 2 ) + 2/3 * ( -3 ) ) = 0.3333 chips ; this is also with the worst possible hole (23o), obviously you'd rather have something better, like even J5 will win 45% of the time against a caller.  (I'm assuming the small blind always folds here).

Don't get too attached to semi-bluffs.  Many people get enamored of this play.  Remember that it only works if they believe you have a hand, not a draw or a bluff !!!  Also make sure you bet large enough to put pressure on them to fold - a small bet is not a semi-bluff since it gives them no chance of folding.  Also make sure you don't overbet, since them you are just getting too much money in the pot with only a 1/3 chance of winning it.  For example, if your stack is much larger than the pot and you move all-in with a flush draw, that's a huge mistake which will burn you in the long run.