as daniel said
The most important reason why itís dangerous to risk all your chips pre-flop is that you simply canít be certain that you are even in a coin flip situation at that time.
You could actually be a much bigger underdog than that. Say youíre playing pocket sixes against a higher pair, like 9-9. The outlook is dismal in this situation as you would have just a 19% chance to win the pot. Thatís not a coin flip; youíre a 4-to-1 underdog!
You see, even a lowly starting hand like pocket sixes is a favorite against a seemingly strong hand like A-K. But the real problem with playing the sixes is that youíre banking on the best case outcome to win the pot before the flop. Either youíre in a coin flip as a small favorite or youíre a massive underdog against a bigger pair.
Rookies often make this mistake. Theyíll overplay these hands before the flop by calling big all-in bets, or even worse, theyíll reraise all-in.
Professionals, on the other hand, dread the idea of shoving all their chips into the pot in coin flip situations. Once they push their chips in, they understand that skill is no longer a factor. Instead, theyíre simply at the mercy of Lady Luck.
Thatís why pros wait to take their chances with coin flips until after the flop.
Itís often said that to win a poker tournament you have to win your fair share of coin flips. Thatís true. But itís not often mentioned that you can choose when to do your gambling. Youíll be much safer to delay moving all-in until after the flop, especially if you hit a two-way draw like a straight and flush draw, a pair and a draw, or a draw and overcards.
Okay, say you hold 4h-6h and the flop comes Jh-6d-3h giving you a pair and a flush draw. Itís impossible to be drawing dead against your opponent in this situation. If he has three-of-a-kind, you can still hit your flush. And if heís on a better flush draw, well, youíre still in the lead with a pair of sixes.
This is the kind of situation where you can gamble aggressively because you canít be a massive underdog. Even if your opponent holds As-Ad, youíd still be the favorite with a 51.6% chance to win. If he has Ah-Kh for the better flush draw, youíd be favored to win a little over 52% of the time. And in the worst possible case, if he has three jacks, youíd still manage to win the pot 30% of the time.
Do what the pros do and wait until after the flop to try to win a race. That way you can be fairly certain that youíre in no worse shape than a coin flip situation. Also, by playing aggressively after the flop, you might actually force your opponent to fold his stronger hand, thus winning the pot without even having to see the next two cards.