Checking the turn.
Here's a good way to find out the strength of your opponents hand within certain circumstances.
You need to be heads up, and you need to be second to act every round. It's a great way to determine if your opponent is trying to make a play on you.
Say you have A-K, and you raised 3x the BB preflop and only got one caller. They're first to act post flop when the flop comes down Q-J-4. Your opponent checks. You've missed the flop but you have a straight draw with 4 outs. You bet into your opponent with a semi-bluff of 4x the big blind. You get called quickly. With two face cards up there it's highly possible he may be trying to make a play on you with his AQ, KQ, or QJ. Flop comes with a 5. Your opponent checks. The 5 is no help to you, but now you're wondering if maybe he is check-calling you, just trying to feed off your bluffs. Check the turn. When the river comes a lot of players, being first to act, will not want to check if they have a hand because you might just check it too. A lot of people who were trying to check-call will now bet into you in hopes of getting a little more money for their hand. They don't want to check again because it's not certain you'll try and bluff again. Lets say the turn is a 7 and so your hand is shot. This time he bets into you because of the play you made and so now you know it would be wise to fold your hand without having lost much.
It's a great way to prevent you from trying to semi-bluff your way to victory in a pot only to end up all-in and getting busted by someone who made a great play.
Hope this helps a lot of you!