Definition of check/raise
The check-raise, also known as c/r, is a powerful weapon for the out-of-position player. The check/raise is the act of checking on the flop or turn or river with the specific intention of raising against any bet. Checking/raise is a calculated plan and should be thought out one move in advance.
There are a plethora of reasons to check/uncheck
The reasons for check/raise are extremely numerous. Let's look at two of them.
-In scenario number 1, you have 7-8 in diamonds on 3-4-2 with two diamonds. You are out of position against an overly aggressive player who is bluffing too much. This is the perfect time to pull a check/raise out of your hat. Since he's still betting, the chances of them folding to your raise are high and you'll win an uncontested pot with a single draw, which is fine, but if he calls and you hit your draw, you'll have built a bigger pot.
Scenario number two is check/raise to protect your hand. You have 99 on Q-J-9 with two hearts. This is an ideal flop to check/raise since your opponent will often bet it and you will play the pot faster. By making a check/raise, you are doing what you need to do, which is to put money in the pot quickly on a dynamic board.