When your girl, preferably wearing a skirt and no undies, hops on your pogo stick while you're driving down the interstate at night. The point is to a) both orgasm before you b) hit a Dodge Durango in the opposite lane.
See Stephen King's Thinner or David Cronenberg's Crash for specific examples.
I was driving back home at 2 a.m. last night and Mildred, out of nowhere, gives me a road pop to wake my ass up. Dude, you gotta believe me. I think I still have some period juice residue to prove it.