Occurs during doggie-style sex when the man is thrusting vigorously into the woman's vagina, then abruptly pulls out and jabs his penis in her anus. The "curb" in this case is the woman's perenium (or "taint"). If the woman complains, the mans feigns innocence and claims it was an accident.
Last night my old lady said she didn't want anal, so I just jumped the curb.
If you jump the curb enough, your girlfriend will figure out you're doing it intentionally.