An ambulance in perfect condition is required. Jump on the step to get in the ambulance, but do not enter it. Keep running (but not sprinting), and jump on the mirror. Immediately jump on the hood of the car. Shoot out the windshield with a hand pistol, then turn 180 degrees. Walk to the edge of the hood, but do not fall off. Get in the dead center of the hood. Get some grenades (make sure you are throwing back on) and aim all the way. Press Fire, then run. If done correctly, it should be doing a backflip on a spin while moving diagonally in the air.