The act of placing the penis between your legs so that it protrudes behind you and then masturbating. You are then able to observe yourself in a mirror and it will look as if you have a minge. You may then imagine that this minge belongs to the female of your choice. This can be handy if no pornography is available and the bishop is asking for a bashing.
I had to wank last night and my wife had chucked all my porn. So I had a back wank and pretend I was looking at Kate Moss.