CSP adjust pitch in function of the torque available to correct the effect of the air momentum. Maintaining a Cte RPM in function of the pilot inputs helps the pilot to concentrate on flying the aircraft and not the engine.
The down side being a slight decrease in perf (at least absolute achievable perfs).
A bit of ggl search give me this one :
http://www.hariguchi.org/flying/info/prop.html
Take a look !