Cycle down through the stances from low crouch on a steep slope.

The sitting position is meant for shooting from a decline slope. You sit facing down the slope, using your knees and arms to stabilise the weapon.

I was taught this in British Army training and it baffles me as to why you would add this stance but not allow it to be used on decline slopes.

Conversely the same position would not work on incline slopes, as you would most definitely roll down the hill backwards!