There's some difference of opinion about what happens next. The kick is supposedly in the sensation of fur. I am skeptical about this whole damn business. I should note that there are nerve endings only in the lower extremities of the rectum, and thus there is nothing to be gained by shoveling extended families of gerbils into your lower quadrant. A word to the wise.
Complications often occur.
Cecil sternly advises caution. And stick to mammals your own size.