If you've ever been with a guide and the fish are a little hesitant to bite, they will gently hit the side of the boat, maybe use an oar or an old rod to tap the floor of the boat to "agitate" the fish. Rumor is, the vibration is like a school of fish feeding and turns them on. So, instead of having to do it manually, it's simply a box that does it that you can adjust the speed of the tapping (thumping) sound. What I have seen with mine is a few fish on the graph and with in 5 mins of turning on the thumper, the size of the school tremendously increase. We tested it a few weeks ago by turning it on and off (4 cycles over a 30 min period). We would go from one person at a time catching, to turning it on and every person in the boat having one or two on. Most guides do it, they just do it themselves verses spending the money to purchase one. They seem to work for most schooling fish, sandies, crappie, striper, ect.
https://www.youtube.com/watch?v=ZddiQ817_X8