When you sit on your hand for an extended period of time then proceed to masturbate. Your hand will have gone numb so it feels like someone else is doing it for you.
Where a man wears a condom whilst masturbating, as opposed to catching the spunk in a tissue/ something similar. The word 'posh' is used to denote the expense of buying a pack of condoms for such a menial reason (to make the wank more pleasurable).