The node is just for accessing the weightmaps, they actually belong to the dForce modifier itself and they modifiy whichever proeprties they apply to (by multiplication - so if the dynamic strength is set to 0 in the surface properties then no weight map can make it non-zero, but if the strength is set to 100% then a weightmap can set the actual strength on a vertex to any value from 0 to 100%).