This is really a sort of sexual joke that teenage boys wish they had enough nerve/skill to execute. It involves cutting a hole in the bottom of the bucket or bag of popcorn through which he would insert his erection. His date would not notice that he was doing this (in the bathroom?) and neither would anybody else in the theatre(back row, no lights?). When she reached in the bucket for some popcorn, she would find a buttery surprise.
Of course in your sich, you would know what was going on and be giving him an exhibitionist fantasy handjob.
"Tell me, what is it you plan to do with your one wild and precious life?" - Mary Oliver
