Semi-bluff
A bluff with a draw behind it: folds win now, outs win later.
Say you're on the button with 8h-7h. The flop comes Kh-9h-2c: a flush draw and nothing else — no pair, no made hand. Villain raised from UTG, got called, and now fires a continuation bet. Instead of calling or folding, you raise. You're behind right now, but nine hearts are left in the deck, worth close to 35% to make the flush by the river. If Villain gives up his ace-high or a weak pair, you take the pot on the spot. If he calls, you're a live dog with real outs.
That's the whole edge: two ways to win instead of one. A pure bluff needs a fold or you lose the pot outright. A semi-bluff collects folds now, and when it gets called it still holds equity — often 30 to 40% with a strong draw — so the raise costs less than it looks. It disguises your hand too: a flush-draw raise looks the same as one made with top pair, which makes your later bets harder to read.
The common mistake is treating weak draws like strong ones. A bare gutshot is four outs — raising it is barely different from a straight bluff, just with better math. A flush draw is nine outs; stack it with an open-ended straight draw and you're near fifteen, sound to raise even into a call. Beginners also size these too small. A tiny raise into a big pot folds nobody out, which throws away the "folds win now" half of the play.