It works like this. A car, usually with a driver and at least one passenger, rear-ends or "bumps" you in traffic. You get out to check the damage. The driver or one of the passengers jumps in your car and drives off.
If you're bumped by another car, look around before you get out. Make sure there are other cars around, check out the car that's rear-ended you and who's in it. If the situation makes you uneasy, stay in the car and insist on moving to a police station or busy, well-lighted area to exchange information.