I'm guessing its the reduction in the amount of rebound your suspension sees (damping). On a perfectly damped system, you should see your suspension oscillate maybe 1 times (2 max) before coming to a complete stop in movement; if you think of it as a graph vs. time ... if you put weight on the rear and release, it should bounce back up (above the regular height), then come back below just a bit and then settle down at the final height. Does yours oscillate for a more than that before coming to rest?
Typically, you want your damping to be like the green line (not really comfortable for ride though), but in reality it's like a mix of the RED for the first rebound and then settle like the GREEN line. The above graph was to show you the difference in oscillations. The below graph is what you're looking for in a balanced system.
EDIT:
Found the perfect graph for you! Look at the number they want you to be above. Less than 0.3 and you see a lot of oscillations and greater movement before coming to a rest .. the higher you go, the quicker you settle. Once again, the green line is perfect but harder to achieve with ever changing loads/etc and the preferred numbers should be between 0.3 and 0.7