Soap before clothes is best. In most washers, the poly tub moves rapidly under the basket, and fins on this tub agitate water around the basket and up through the load. The basket and agitator work to rotate the clothes in a "torus" of sorts, usually downward in the center. If you pour soap directly on clothes, it will take a while for the "torus" to release all the soap, as opposed to having the soapy water permeate it through the tub's agitation. This is more apparent with full loads, where full concentration might not be reached until late in the wash cycle.