It will be because you are trying to turn the wheel, which is connected to the diff, which is trying to turn the other wheel, which is on he ground, so then it will try to turn the prop, which will try to turn the gearbox output shaft, which is geared to the main shaft, which is connected to the input shaft, which is held in place by the clutch, which is connected to the engine

So you can move the wheel to take up all the slack until you are then trying to turn the engine with that wheel

If you are not in gear the output shaft is not locked to the countershaft in the box so both will rotate freely and let you turn the wheel