Think of the box as a manual box with power assistance when more than a preset amount of pressure on the steering wheel is applied. This is what gives you "road feel" when driving straight ahead and when light input on the steering wheel is used for correction on slight bends etc, there is no power assistance. This preset amount is controled by the torsion bar in the input shaft in the box, connected to the steering wheel. When weight on the steering wheel is high enough to flex the torsion bar, this opens valves in the rotary valve block to flow oil to the piston to give power assistance. So the torsion bar controls how hard you need to turn the steering wheel before the assistance comes in. If the bar is weak or broken, or parts in the rotary valve block are slogged out the assistance will come in too early, and you will have too light a steering with no feel and be able to spin the steering wheel from lock to lock while stationary with a flick of the wheel.
A picture from the manual is worth a thousand words...........
https://www.aulro.com/afvb/images/im...008/12/577.jpg
https://www.aulro.com/afvb/images/im...008/12/578.jpg

