Hey I'm Mickey Numbers. I'm a professional poker player and I'm going to explain semi-bluffing to you. A semi-bluff is an aggressive action that is taken without having a strong made hand but rather having a draw, like an open ended straight draw or a flush draw; a hand that has a lot of value in that it's going to improve a significant amount of the time on the next streak. So if we're on the flop, it's going to improve to a strong made hand on the turn regularly.
If we were on the turn, it would improve to a strong made hand on the river regularly. Typically, semi-bluffing can be thought of taking aggressive action when you have 8 outs or more. A semi-bluff is significantly different from a stone cold bluff, which is a bluff with no hand no draw. Take a look at this situation where you have 8-3 offsuit on a board of 9-7-King, 2 to the flush. If you bet at this pot or raise a bet on this flop and you fail and your opponent chooses to continue in the hand, 8-3 offsuit could often be drawing slim or even dead to your opponent's hand. However, a hand, like 2-4 of clubs, has a flush draw.
There's 13 clubs in the deck; 2 of them are on the board and 2 of them are in your hand. That means that there's 9 remaining clubs. When you have 9 clubs to make a flush, you have 9 outs. This is a classic semi-bluffing hand here. Although you have a weak flush draw, flushes are usually strong when you make them using both hole cards. Typically, you want to take those hands to showdown and play a big pot.
So if you take an aggressive action at this flop and your opponent continues in the hand, you're not just out of it like you would be 8-3 offsuit, where the overwhelming majority of the time you're not going to be able to continue past the turn. In this case, you're going to hit a flush on the turn about 18% of the time and when you don't, you have another shot to hit it on the river. When you're taking aggressive actions at the poker table and you're bluffing, ideally, you want to be semi-bluffing, where if your bluff fails, you have a back up.