when you're making out with a chick, and you slide your hands down her pants and pinch her bare ass. UNDERBITE!!! see the intruder...
i gave my roommate's mom the underbite last night.
1. while making out with a chick, you slide your hands down her pants and put your digits in her vagina.
2. reverse underbite with penetration.
my date was going bad until i gave her the intruder. bitch was loving it, yo.