The zipper is a two handed sex move performed on a penis by someone else. Basically, while kneeling in front of your partner, you interlace your fingers grabbing the penis with both hands and rhythmically stroking it in a back and forth manner. This hand position is superior due to the added friction and position of the hands to apply more pressure with least amount of effort.

When you eat dirt for the first time and leave your family to build a shack in the woods