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 crankmotion 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
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.