When a guy puts some mashed potatoes in a plastic bag, puts his cock in the bag, and then proceeds to pound his lady, sticking his mashed potato-covered dick inside her vagina. This prevents conception since the vagina is protected from the cock by not only the plastic bag but the potatoes as well.

It can also be done with toothpaste, whipped cream, or other white airy materials instead of potatoes
Dude 1: Hey I got some potatoes

Dude 2: Tiime for the ol' Snowball Express
by thesnowballexpress May 17, 2011
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