There is one big exception to the check-raising rule: If you think you can get a free card by doing it, it’s worth trying. This exception is easiest to describe by example.
The free card setup
Typically, you want to try for a free card if you’re trying to make a straight or flush draw.
Say you’re playing J♦ 10♦ out of a late position, and there was one flat-caller pre-flop from a middle position — meaning, three people are still in: you, the flat-caller, and the big blind.
The flop is Q♦ 5♣ 2♦.
The big blind checked, the middle position bet, and the action is on you. You have a flush draw here, but most likely you don’t have the best current hand (all it takes is one player with a card bigger than a jack). A call might barely give you pot odds.
But if you raise here, what’s likely to happen?
Both players could fold and you win. That’s pretty great.
One player could reraise you, in which case you know she has some form of a smokin’-good hand (either trips or an A-J in this situation). Assuming you don’t have pot odds for drawing your flush, you could fold.
Most likely, one or both players will call. They perceive you as having a good hand (or maybe just trying to bluff), but the flop has slightly fit their hands as well. They call you, waiting to see the turn.
And here is where the beautiful part happens. On the turn, anyone still in the hand checks to you. If that player has made a hand, he’s going to wait for you to bet so he can check-raise you. If he missed, or if he’s still just in a so-so position, he’s waiting for you to bet and then he’ll make a decision on what to do. But because you’re last to act, you don’t have to bet. If you miss your card on the draw, you simply check and the dealer will summarily bang out the river card. If you made your hand, you just go ahead and bet.
What this effectively means is that you get a free river card. And you saw the turn for half-price, because your raise was less than a call on the turn would have been.
Defending against a free card
Now that you know the trick to get a free card, you can also defend against it. Any time on the flop that you see a raise behind you, with what appears to be a board draw — again, flushes and straights — you can either reraise before the turn if you have a very good hand, or simply call and then bet (instead of checking) when it’s your action on the turn.
Notice that we’re talking about draws here, not made hands. Raising into a person with an all-club board showing is foolish at best, and suicidal at worst.