It's the amount of shock that is compressed for wich inch of rear wheel travel. In car engineering it's called Instalation Ratio, in bike terms is the opposite of the leverage ratio applied on the shock. In leigh terms, because the curve ramps up means that the shock is easier to compress at the beggining and becomes increasingly harder to compress through shock travel to avoid bottoming.

 Buy linkage software! it's 25$ and you can do this and much more.
http://www.bikechecker.com/

 rate of compression