Bluffing and Position
In most instances, acting last – after you’ve had a chance to see what your opponents do – is a big advantage. But when you’re bluffing its often advantageous to act first.
However, if your opponent checks and you bet, he’s likely to realize that you are trying to take advantage of the fact that he’s shown weakness. As a result, he is more likely to call – or even raise, if he’s a very aggressive player - with marginal hands.
But betting from first position converys the image that you really do have a strong hand. After all, you are betting into omeone who could have a really powerful hand. Your opponent, of course, will realize that and be more prone to release a marginal hand than he would be if he checked and then you bet.