Rhyming slang for Yank or Wank {which is the cut down version of wanker}which turns out to be the same thing
Imagine doing the hand crank motion with your hand whilst thinking of a yank ....see wank and yank = the same thing ;o) who says us limeys dont rule the waves :0)
When your penis is masturbated by your partners hand in the inverted position to which you would normally pleasure yourself. I.e the hand would by placed thumb side down closest to the base of the shaft. Most commonly
