The semi-bluff occurs when you have the possibility of making a winning hand but decide to bet prior to that possibility becoming a reality. You're betting on pure potential, but the value of the semi bluff bet is that it gives you an extra way to win the pot. Your opponents might all capitulate then and there enabling you to win the hand with nothing. Or, they might call you down only to see that you hit your draw down the line and still beat them. Both scenarios are incredibly gratifying.
In order to become an effective poker player at any level beyond that of novice it is imperative that you not only understand this concept but that it becomes a standard weapon at your disposal. If your only way of winning a pot is via good cards you will lose all your money playing poker. You are far too reliant on good fortune, and good fortune only smiles so often on any one poker player. But if you have the ability to visualize winning a hand and you can convince others of the accuracy of that vision, then you are in with a chance of booking a win at the table.