A 'bump start' converts forward motion/momentum of a vehicle to engine cranking through a standard transmission and clutch.
Typically, a stalled vehicle which cannot be started with conventional means would be pushed forward or rolled down a hill in neutral then put into a forward motion gear and the clutch would be engaged. This has the effect of converting the forward motion of the vehicle to the engine like a pseudo-starter motor; essentially reversing the conventional angular momentum from motor-to-wheels to wheels-to-motor.
If the vehicle was on a slope that dictated rolling in reverse, the same process could be accomplished by rolling downhill backwards before engaging the clutch while the transmission was engaged in reverse. Given that reverse gear is typically a non-synchronous gear the vehicle would have to be in reverse (not neutral) before using gravity to facilitate the rolling motion. This is not as efficient (and offers less options) as rolling in a forward direction.
