I had a friend who actually bought a vibrator and messed with relays, etc.. (don't know exactly what was involved) and wired the thing into his tach so the more revs, the more vibration he gets. The idea was to get a chic on his bike and get her "revved" up so he can get laid easier.
However, there was one compllication with his grand plan - no woman ever got on his passenger seat.
Just a minor detail.