When I was young my parents took me to Amity Island, we had a summer rental. There was an old fishing boat down at the marina, I think the Orca... that had a contraption that looked similiar to that, the Captain Quint was a real a$$ but I asked him what it was used for. He told me it was used to kinda sandwich whole mossbunker to let drift behind the boat, and when used along with chumming.... It's actually an effective way to attract sharks.