Semi bluff.

The added equity of winning the pot with the all in raise on the flop is what makes this a winning play. I make this play sometimes, but usually only when I have something in addition to my flush draw, such as middle or bottom pair or a gutshot. Having 13 or 14 outs on the flop actually makes you a favourite a lot of the time when you do get called. That makes this play a big money winner.