A passenger motorcycle seat addendum where extra seat-foam padding is added to the seat to provide a protrusion for the passenger (usually a female), to push against with her groin under hard braking. Usually used in conjunction with having her hands placed on the tank while her boyfriend/husband acts like a hooligan in traffic.
"That vagina knuckle really saved your lower back when that mini-van pulled in front of us and you had to slam on the brakes."
by Spooph October 1, 2009