Think it has something to do with torque multiplication. assuming a 3 to 1 1st gear and a 1 to 1 first gear with the engine driving the input shaft the rear sees a 3 to 1 torque multiplication but with the rear wheels driving the input shaft it there would be a 1/3 torque multiplication divide by 3) When in 4th gear (1:1) and the input shaft has the same torque (i.e. more likely to turn the engine over). When driving, downshifting offers braking.

I think