Originally Posted by nick k
The other thing with lifting the rod is that some of the pressure of the set is actually absorbed by the flex of the rod. A rods entire job is to absorb pressure, and this isn't always what you want. Strip striking ensures that every pound you pull is transferred directly into the strike.
A secondary reason (and some people consider this the main reason) is that a strip strike doesn't always cost you the fish if it is ill timed. If you lift the rod to strike, the fly moved very unnaturally and in many cases comes clear out of the water. Strip striking simply looks like a quick dart of the fish to avoid being eaten, and in many cases will not deter the fish from striking again. This is the primary reason I do it.
Nick's explanation is the primary reason I use the strip strike. If you lift a nine-foot rod with a quick twitch, that fly is moving a considerable distance up and away from the fish. The fly often comes out of the water toward you, and you're now out of the game. If you continue to strip, those short striking fish will continue to follow and try to grab your fly. I've had many occasions with four or five bites in one cast before the fish makes the commitment and I hook up with him. Had I lifted the rod instead of using a strip strike, I would have pulled the fly out of play, missed the fish, and been out of the game until the next cast.