[Rust-O-Matic]

Dumb question... What is this switch for that was mounted to the intake on my small block next to the carburetor? It was was hooked to the throttle linkage with a piece of fine cable, which appeared to pull the switch closed at full throttle. The black wire attached to the switch in the pic was just grounded to the bolt holding the switch to the manifold. I don't get it. Any ideas?