Ooohh, nice idea, from another devious mind.  Especially if that dildo was attached to a fucking machine, or the kinky creation I made a while back...which I referred to as the "drilldo". Basically a dildo mounted on a vac-u-lock adapter, which was then inserted into a drill. After forcing the dildo into the sub's ass, one pull of the trigger would make the dildo spin inside his ass, at variable speeds depending on how much you pull the drill trigger. Think like you're operating a drill to drill a hole or to drive a screw into a hole. Except in this case the drill bit or screw is the dildo, and the hole...is the sub's hole. (evil grin!)