From my perspective, this is one of the most important aspects of one's poker game, especially when you find yourself out of position. Here's how it works and what to watch out for...
It's $1/$2 NL and you've got $200 in front of you. The rest of the table (10 handed) ranges from $60 to $400. So far the play is fairly loose-aggressive across the board, fairly typical.
You're dealt AQc, two off the button. Two of the usual suspects limp in, the rest fold, and it's around to you. This is a solid hand and you've got decent position, so you raise to $15. (The pot before your bet is $7, so you picked $15 to isolate against loose players without overbetting the pot too much.) The button calls, blinds fold, one of the two limpers calls, and we're ready for a flop:
2s 7c 8d
The limper checks to you. You look at the board. Could well be a blank flop, but you wouldn't put it past the button to be playing something silly like A7 or J8. He's been raising from the button all night and clearly loves having position. If you check to him, there's a 95% chance he'll bet at it whether or not he's got anything. You need more information than that - and nothing buys information like money.
The pot is an even $50, so you decide a half-pot bet of $25 is the way to go... Once again, you want to be aggressive enough to get the information you're looking for without putting too much into the pot. Too small of a bet and their call won't tell you anything. Too large of a bet and you're risking too much without a made hand.
The button calls. The limper folds. At this point, because the button's not a complete idiot, you can guess he's got one of three things: a halway decent draw (like 9T), a semi-made hand (middle pair, weak top pair, etc), or he's hit something big and is giving you just enough rope to hang yourself.
Now for the turn:
(2s 7c 8d) Jc
The plot thickens. One of the potential straight draws hit. If the button was playing something like J8, like you thought he might be, that's two pair and takes away some of your outs. On the other hand, there are two clubs on the board now and you've got a draw to the nut flush.
The pot is $100. You bet $40 to keep the pressure on without investing too much of your stack. You've invested $80 of your $200 now. The button has you covered.
After some thought, he calls. Don't read too much into the hesitation, just go on the information you've garnered from the bets. It's becoming less and less likely he's got a semi-made hand. It's getting a little expensive for that now. Maybe he's still on a draw. If he has a straight, a set, or two pair, it's extremely risky of him to simply call you here and risk the flush coming out... Especially since he's been aggressive all night, you would suspect he would take the opportunity to raise you and try to take the pot now.
But, regardless, it's time for the river:
(2s 7c 8d Jc) 2c
There's your nut flush. The hand is probably yours, but keep your eyes open. The paired board means he could have hit something bigger here - if he was willing to risk the flush draw with his set or two pair.
You weigh your options. The pot is $180 and you've got $120 left. Any reasonable bet and you're committed. But... Maybe shoving still isn't the best option. If he's missed the lower end of the straight draw or something else silly, he might be coaxed into a mistake by a smaller bet... If he's got you, there's not a ton you can do about that, but you really think he would have bet on the turn with anything that could beat you now.
You bet $60 into the $180 pot. It's a fairly small bet, so he might call it with something questionable (like trip 2s) or even shove because he thinks your river bet shows weakness. At the very least, he's unlikely to lay it down for $60 more.
He shoves. You'd already predetermined your call.
He shows 8Tc. He'd flopped a questionable top pair, turned the flush draw, and rivered a weaker flush. You double up. (If you hadn't had the guts to put a bet out on the flop, he would have bet the pair and you would have had to lay it down with insufficient information.)
Okay, I hear you saying it in your head... It's easy to set up a fictional hand and knock down an imaginary opponent. Yes, that's true, but you'd be surprised how often situations like that occur. You could have hit a higher pair or any number of better hands... But the important thing is, you had the courage to put some chips out there, wisely, to fish for information.
Yeah, you say, but that's an easy strategy to defeat... All he had to do was reraise me and I'd have to bail, giving him those chips for free because I bet without a hand.
Well, yes, that does happen... But that brings us to the second part of the "Feeler Bet" strategy. When you hit something big, put out a feeler bet. It helps to build the pot and DOESN'T give them much information - because you use this bet to control pot size (more on that at a later date), feel for information, AND to entice a raise.
Same situation... Except this time you've got 6s6h two off the button and raise to $15 just like you did before.
For the sake of the argument, we'll say it's the button who calls you again. He's steaming a little bit over how you "caught that flush" on him earlier (see: POKER DRIVEL: Play for Yourself) and wants to catch a hand to take "his" money back.
Flop is Ks 4c 6d.
You bet $25.
He's "not going to let you chase him down again", so he goes all-in with his remaining $110 with KhJs.
You kick his butt, take his money, and send him packing.
He acted on semi-reasonable information. He saw you do that earlier when the flop hadn't hit you... And you used the art of the feeler bet combo to misdirect and defeat your opponent.
One last thing as I wrap up... As you know, events unfolding exactly as I've written them above is rare. Often you'll take the pot with your post-flop bet, given it's a fairly blank flop and fairly likely no one hit anything worth calling. Sometimes you'll get reraised and you'll have to bite the bullet and lay it down. But if you utilize this strategy correctly, it should do well for you in the long run.
Try it out and see for yourself.