It is usually used with a very good pre-flop hand such as pocket Aces on Kings. Instead of betting big, the slow player will sit back and limp in (call or check) as if he doesn't have a hand worth betting, hoping that an opponent will have caught a lower pair or a straight draw and will come in with a big bet.
It is a classic poker manouever and one which has won a lot of money for poker players down through the ages, but it has lost it's effectiveness. The simple reason for this is that everybody is now slow playing their big hands so it is almost expected.
So how do you use this situation to your advantage?
If you catch a big hand, you need to come out betting. Instead of trying to tempt your opponents into making a move so that you can come in over the top, make the bet yourself. Now most amateur players baulk at the thought of this in case the rest of the table folds and they "waste" their big hand, but in reality you will normally always get some betting action from somebody.
By betting your big hand instead of slow playing it, you are performing a double bluff since most people would slow play it. Your opponents will often assume you are bluffing and you will get some betting action and ultimately win more money.
The other reason you should bet is that if you don't bet, you are allowing your opponents a free look at the cards and every time a card is turned over they could be converting a losing hand to a winning one.
There is only one situation where it will always pay to slow play your hand - when you flop an absolute monster that almost cannot be beaten. For example say you are dealt Ace, Four of clubs and the flop is Queen, Ten, Nine of clubs. You have just flopped the nut flush and you're pretty much unbeatable. OK somebody could have a pair of Queens, Tens or Nines and catch a fourth on the turn or river, or they could be holding the King of clubs and catch the Jack for a straight flush, but the chances of you not winning are now remote.
This is the time to slow play your hand. There are all sorts of hands that somebody else might be holding to make them think they are in a good position here; two other clubs, a straight draw, triples or even just a Queen for the top pair, whatever they've got the chances are somebody will think it's worth betting. If the flop or river produces a King, Queen Jack, Ten or Nine you might get a big raise or even an all-in from somebody.
Now you let them build the pot up by just calling the bets and take down the pot with your nut flush.
