If you’ve ever watched the scene where Garth is working on the mechanical hand in the studio and wondered what in the heck that was all about, it’s because it’s part of a backstory we weren’t privy to. According to Dana Carvey, Garth had kind of caught on to what Rob Lowe’s character was up to - exploiting the guys and going after Wayne’s girl. So he devised this mechanical hand to kill Benjamin and was working on it when Benjamin himself walked over, sending the hand into a frenzy. Garth was worried that the hand was going to kill him right then and there in front of everyone, so he had to bash it to death with a hammer before it strangled Benjamin. So now you know.
— Movie Trivia: Wayne’s World. Zang! - Neatorama
