I'll have a stab for sh*ts and giggles
Controls the rate at which the spring compresses when an upward force is applied to the wheel - say a bump in the road
Controls the rate at which the spring expands when the wheel comes across (say) - a hole in the road
I'm happy to delete this reply in it's entirety if I have that wrong!
The damper will have a position where it is sitting during travel along a smooth surface, shall we say mid stroke.
When you hit a bump for example (it could have been a depression), the damper will be forced to compress somewhat until the bump is cleared, then the spring will force the wheel back down to the road surface. Note that the spring will slam the wheel back down faster than it would have fallen under itís own weight.
But without damping, the suspension will theoretically try to oscillate to and fro, past that idealised mid point that we have in our example, due to the inertia in the mass of the suspension components.
If we leave the bike for a moment so as to not be distracted by all the stuff going on in the reality of riding.
Again if we have our 200 mm spring out on the bench disassembled from the damper.
If you were to compress that spring say 50 mm then release it suddenly, high speed photography would show you that it recoils out PAST the 200 mm point to say 210 mm then back to 180 then out to 205 ....etc
That is an idealised set of numbers and possibly they would be a lot less, but the point is, the spring would oscillate around that mid point until enough WORK had been done for it to come to rest.
It wonít take long because the spring has low mass and therefore low kinetic energy.
In the reality of our heavy suspension components, they will try to oscillate for a longer time up and down each side of our mid point until the energy is dissipated, this is why we have our damping.
It forces the spring to act against a hydraulic resistance after the bump has been cleared and you are back on smooth road, so that oscillation is quickly reduced to zero.
Imagine you had a bike cranked up clear of the floor with the damper removed from the rear shock(s).
Compress the suspension to itís physical limit by some apparatus, the release it suddenly.
Youíd see the spring fire the swingarm back out to the normal position but the energy stored would take it past until the spring was actually STRETCHING, then that oscillation around the mid point would commence.
The work required in moving fluid through the ports of a damping unit is what dissipates that kinetic energy.
To directly address your suggestion.
If you run over a six inch high speed hump for example..
The primary task of the damper is not to control the RATE that the suspension compresses, because unless the suspension can compress to ALL of that six inches in the instant that you hit the bump the shortfall will be transferred into the frame and YOU will go up by the residual amount.
Again the PRIMARY function of the damper is to dissipate the residual energy in the system by compelling the spring to do WORK in the pumping of the fluid through the restrictions in the shock body.