the thrust is transferred thru the leaf springs. because of inertia the axle will try to stay in place and rotate. when the springs start ot bend and match the resistance of the twisting force then the wheels start to turn. also the rubber bushings in the spring eyes compress. The reason racers use ladder or traction bars is to eliminate all that rotation and compression. .. with weak shocks or where they are mounted you can get a harmonic movement or po-go effect and the rear end will bounce. the 50 's style single traction bar was effective when the spring bushings were changed to be stiffer and the spring leaves had additional clamps. depending wher ladder bars are attached to the frame you can also get some front end lift.