The Behavior Modification System is designed to help those who has a habit of cursing a lot. According to some dialogue from Buster, it also works on nail-biters.
When Fowlmouth asks Buster to help him stop cursing in order for him to ask Shirley to the dance, Buster uses the System to cure Fowlmouth's habit. Unfortunately, Fowlmouth curses the entire time, to the point where it breaks the machine. The machien appears again at the end of the cartoon when Buster lets out a (bleeped) explicit. The now fixed system does the same treatment it did to Fowlmouth earlier in the cartoon (seemingly to no effect, as Buster curses the entire time as well).