Similar to the Vulcan, and the shocker. The rusty chainsaw is the act of penetrating a womans' anus with the entire hand very rapidly. The remnants left on your hand after the act give the "rusty" appearance.
My wife pissed me off the other day so, as revenge when we were going at it doggystyle I gave that bitch the ol' rusty chainsaw. That's why I'm staying in the hotel tonight.