When one female folds her arm down, as one male proceeds to slide his carefully lubed erection in between said fold, pointing towards the armpit. The male then continues to repeat this process, focusing on rubbing the bell of his penis against the female's axilla. It is often best for the female to have a well shaven axilla, as it can become extremely abrasive for the male genetalia.
Winston: "I say, Margaret, what say you to a randy romp of a good ol' fashioned cockpit?"
Margaret: "That sounds quit agreeable, noble sir, but I must shave my oxter, for it could become quite unagreeable with your tallywhacker."
Winston: "Indeed, then it's agreed! We shall have a cockpit after tea!"