In most cases, aggressive play is winning play online poker. You should constantly be applying pressure to the other online poker players to give them tough decisions to make. You may reraise when you think you're either beaten badly or your opponent is bluffing. Bluffing and semi-bluffing are important to keep yourself unpredictable, and because you're keeping track of the ranges of plausible hands, it's quite likely you'll often know where your opponent stands.
Obviously, then, a semi-bluff cannot occur unless there are more cards to come. When you bet as a semi-bluff, you are rooting to win right there just as you are when you make a pure bluff. However, in contrast to a pure bluff, you still retain a chance of outdrawing your opponent if you are called. Even when you bet with a legitimate hand, you are generally rooting to win instantly, but when you semi-bluff, you especially want your opponents to fold because one of them may be folding the best hand.
Cold bluffing is usually restricted to the river, where you might bet into one or two opponents (who might fold) if you have no chance of winning the pot at a showdown. Semi-bluffing is betting with a hand that is not likely best but has some big outs. Your opponent may fold immediately, and, if not, you may hit your out and your opponent may seriously misread you. There is an important balance here. You must have sufficiently tight hand selection criteria such that when you do bet your opponent is positively terrified that you may have a big hand like an overpair. Semi-bluffing is very powerful, because you've been so careful in choosing your starting hands that even if you aren't there yet, you are likely to get there.
The semi-bluff is one of the least understood tools of online poker game, yet it is a very valuable and potent weapon. All professional players use it, and it may be used in any game. It may be a bet, a raise, or even a check-raise. Essentially you are representing a bigger hand than you actually have; however, in contrast to a pure bluff, your hand must have some chances of improving to the best hand.