In poker, to bluff is to bet or raise with an inferior hand. By extension, the terms are often used outside the context of poker to describe the acts of pretending knowledge one does not have, or making threats one cannot execute.
Pure bluff
A pure bluff, or stone-cold bluff, is a bet or raise with an inferior hand that has little or no chance of improving. A player making a pure bluff believes he can only win the pot if all opponents fold. The pot odds for a bluff are the ratio of the size of the bluff to the pot. A pure bluff has a positive expectation (will be profitable in the long run) when the probability of being called by an opponent is lower than the pot odds for the bluff.
For example, suppose that after all the cards are out, a player holding a busted drawing hand decides that the only way to win the pot is to make a pure bluff. If the player bets the size of the pot on a pure bluff, the bluff will have a positive expectation if the probability of being called is less than 50%. Note, however, that the opponent may also consider the pot odds when deciding whether to call. In this example, the opponent will be facing 2-to-1 pot odds for the call. The opponent will have a positive expectation for calling the bluff if the opponent believes the probability the player is bluffing is at least 33%.
Semi-bluff
In games with multiple betting rounds, to bluff on one round with an inferior or drawing hand that might improve in a later round is called a semi-bluff. A player making a semi-bluff can win the pot two different ways: by all opponents folding immediately or by catching a card to improve the player's hand. In some cases a player may be on a draw but with odds strong enough that he is favored to win the hand. In this case his bet is not classified as a semi-bluff even though his bet may force opponents to fold hands with better current strength.
For example, a player in a stud poker game with four spade-suited cards showing (but none among their downcards) on the penultimate round might raise, hoping that his opponents believe he already has a flush. If his bluff fails and he is called, he still might be dealt a spade on the final card and win the showdown (or he might be dealt another non-spade and try his bluff again, in which case it is a pure bluff on the final round rather than a semi-bluff).
Bluffing circumstances
Bluffing may be more effective in some circumstances than others. Bluffs have a higher expectation when the probability of being called decreases. Several game circumstances may decrease the probability of being called (and increase the profitability of the bluff). For example if there are fewer opponents who must fold the bluff, if the bluff provides less favorable pot odds to opponents for a call, in the case of a scare card (a card dealt face up) which comes and increases the number of superior hand they are representing with the bluff. Also the opponent’s betting pattern suggests the opponent may have a marginal hand that is vulnerable to a greater number of potential superior hands. Another bluffing circumstance would be if the opponent's betting pattern suggests the opponent may have a drawing (a poker player is drawing if he has a hand that is incomplete and needs further cards to become valuable) hand and the bluff provides unfavorable pot odds to the opponent for chasing the draw. Another circumstance would be if the opponents are not irrationally commited to the plot and if they are sufficiently skilled and they are paying sufficient attention.
Optimal bluffing frequency
If a player bluffs too infrequently, observant opponents will recognize that the player is betting for value and will only call with very strong hands or with drawing hands when they are receiving favorable pot odds. If a player bluffs too frequently, observant opponents snap-off his bluffs by calling or reraising. Occasional bluffing disguises not just the hands a player is bluffing with, but also his legitimate hands that opponents may think he may be bluffing with. Mathematically, the optimal bluffing strategy is to bluff in such a way that the chances against your bluffing are identical to the pot odds your opponent is getting.
Optimal bluffing also requires that the bluffs must be performed in such a manner that opponents cannot tell when a player is bluffing or not. To prevent bluffs from occurring in a predictable pattern, game theory suggests the use of a randomizing agent to determine whether to bluff. For example, a player might use the colors of his hidden cards, the second hand on his watch, or some other unpredictable mechanism to determine whether to bluff.
So, when should you bluff?
When there aren’t many other players in a pot
Simply put, it's easier to trick a couple people than a crowd. With fewer hands out there, chances are better that no one has made a reasonable hand. This is fairly common though, so many players won't believe you. Some will stay in the hand for the next hand just to "keep you honest", so sometimes this needs to be a persistent bluff over a period of two or three betting rounds. That can be costly if they don't fall for it. You need to know the players before you use this type of bluff.
When you're up against fairly tight players
Those that tend to fold easily are the biggest targets of a bluff. Bets will be put out just as a form of information gathering on this player's hand. If you bluff early (pre-flop, flop) against a very tight player and they don't buckle, you should think twice about trying it again on a future round. They have something. Your job is to determine whether they have a made or drawing hand. Once again, you need to know the players.
You're in late position and everyone else checked
This one you'll have to gauge for yourself. It will most likely force some players out, but not all. This is a pretty common bluff once again, and many players will stay in just because of bet odds, and/or to once again "keep you honest". This is another example of a bluff that needs to be more persistent over a couple betting rounds.
You bet pre-flop and missed
A bluff here can work because they don't know that you have missed! This can be dangerous, and you really have to evaluate to board before you get into this one. Sometimes it's good to bluff when A-K misses, sometimes when 9-9 misses. You have to really feel this one out.
When there is a pair on the board
This is especially useful when the pair is 8-8 or lower. Chances are that these cards might have been folded or are still in the deck. This is one situation where you want to evaluate the hand very carefully if they do call though. This is a great situation to read tells of the players who are NOT involved in the game. It's much easier to give away the fact that you HAD a card than if you HAVE it. Keep in mind that these are pretty common reasons to bluff. Many players know these reasons. Most of the times its just not going to work. The main thing is always to know your opponents and not to bluff so often that it never works.
Knowing your opponents is very important in a poker game, and especially in a bluff. Don’t use this strategy if you don’t know your opponents, as there are a lot of chances this might not work.
