What your friend is talking about is called engine braking. When you downshift without using the brakes to slow the car down the engine will naturally cause the car to slow as rpm drop.I do this often and when I downshift I typically dwonshift aroudn 3500 rpm. I drive a 2000 Honda Civic Si.Your gearing will ultimtely factor is in your engine brake effectiveness.However engine braking is not the fastest way to slow down when used alone. To effectivelt slwo the car the fastest possible way is to use the hyudraulic brakes.