Semi-automatic reels have a lever that comes off the front of the reel where you can squeeze it with your rod hand while holding the rod, and each time you squeeze it line is retrieved. But unlike automatic reels, there is no spring. They are gear-and-ball-bearing actuated. So one pump of the lever = only so much line retrieved. They are far superior in performance and durability to automatic reels. And they are a vital piece of adaptive equipment to someone who wants to fly fish, but only has use of one arm.