Never never get under any car only supported by a jack (even a Benz). Put it in park. Set the parking brake and you should be OK, unless the parking brake is not holding. The car should not move if the brake is holding. Just using park doesn't do it b/c the car can move up to 6 inches or more in park. Just try it on a slight incline. Step on the brake, put it in park and release the brake pedal. It's an education my friend.