A question: if the ambulance is in reverse gear on the final hill, how come it rolled downhill?
If you mean when Sylvia Syms let go of the gear handle, it was facing forwards so would have rolled backwards anyway. On the successful hill climb it was facing backwards.
shareOn any manual geared vehicle the reverse gear is the lowest ratio gear, in this case with the spark plugs removed to make it easier to crank. If Silvia Sims let go of the crank handle without the brakes being on then the vehicle could have easily rolled back down the hill but certainly not so fast as it did in the movie. If the clutch was in as it must have been for the crank handle to move the engine and in turn the whole vehicle via the gearbox.
I have tried moving a vehicle with the plugs out in first gear using the starter motor and it works quite well. There are no crank handles on modern vehicles, they were there a left over from having no starter motor to rotate the engine to start it.
When making a movie sometimes licence is used with the laws of physics.