Opponents will often “fork” their ranges pre-flop. By fork I mean they’ll divide their hand sets pre-flop into two or more sets by choosing different actions for each. The most common way is limping with some hands and raising with others. Also, against a raise, they’ll call with some hands and 3-bet with others. (It’s usually correct to fork your ranges in the second scenario, though weak players do a poor job of choosing which hands go in which range.) A subtler player forks ranges pre-flop through bet sizing. He’ll raise to $20 with some hands, but raise to $30 with others. (This is a very bad idea. Don’t do this.)
Forking is also a bad habit because it’s difficult to create two different sets of hands where both sets will play well on a wide variety of flops. If you raise some hands but just call with others, you want the raising hands to be stronger on average than the calling hands. But you’ll also want a mix of hands in each range so it’s never too obvious which hands you’re playing in which range should you miss a flop.
This, incidentally, is one of many reasons I advise against limping pre-flop. Players who limp generally fork their ranges so they’re easy to exploit on certain kinds of flops. (Yet, you can’t help forking when deciding between calling a raise and 3-betting.)
When your opponent has a forked range, typically due to poor range construction, each forked range will perform poorly on a prescribed set of flops that either do or don’t have “key cards.”
Let’s explore with a simple example. Two players limp, and an aggressive player limps the button. You check the big blind. The flop comes A♣9♣6♠. You check, and both limpers check. The button bets $20.
This flop contains an ace, which in this case is a key card. Aggressive players will usually raise the button after limpers, who typically limp low-card hands like 8♠5♠ or 2♦2♣. It would be odd for an aggressive player to limp the button with a hand that contains an ace. Therefore, the button’s limping range performs poorly on most flops that contain an ace—and therefore he should rarely bet these flops. There’s a good chance the bet is weak, and you should check-raise (or check-call, and plan to bet a later street) with impunity.
This example relies on the aggressive button player forking his range poorly—raising most hands, but still wanting to limp a few of his weak hands. Here, he did indeed fork his range, and didn’t tell a coherent story on the flop when he bet.
But these “key-card flops” can go the other way too and thwart a player’s strong hand range.
Say a player limps, and you raise to $20 with 9♥8♥ from the button. A player reraises you from the big blind to $50. You know this player well enough to know that he will reraise only with hands J-J, A-Q, and stronger. Despite this strong range, you call the $30 because both of you have over $1,000 behind, and you know you can exploit the information you have about your opponent’s range on an array of boards.
The flop comes 7♣5♣4♠, giving you a gutshot. This board lacks a key card (an ace, king, or queen to pair your opponent’s A-K and A-Q hands). Furthermore, it features a possible straight, and a straight-and-flush draw your opponent likely doesn’t have. You, on the other hand, as a button raiser and 3-bet caller, can hold an assortment of hands that hit this flop well.
You’ll typically want to see this hand through at least to the turn, and you can use the threat of having completed a hand to put pressure on your opponent.