To kick someone in the crotch with your lower leg, causing your foot to stick out on their other side. The ideal Iron Pony should lift the person off of their feet. Saying "yee-haw!" while doing this is a good bonus but isn't absolutely necessary.
"If you don't quit it I'm gonna give you an Iron Pony!" or "If you don't want to take a ride on the Iron Pony you better shut up."