When conditions are perfect (e.g. You are sitting on a flat to semi-concave, non-porous surface) and you release a fart that cannot immediately escape the confines of your ass cheeks. The fart will remain in place until you shift your weight or raise at least one ass cheek to "pop" the blister.
I feel weird telling you this, but I find fart blisters sexually arousing
by Jdubyalker December 27, 2012