Knee of the standing foot moves foreward with the body. As the knee of the standing foot bends, just a little when walking, we create compresion and with the help of gravity the free leg is swung forward(by the hips) and into place just in time to collect our body weight. We do not reach with the free foot(hence,we are not using any muscles of the free leg. When we lower more in order to dance we then have to use some muscle in the free foot in order to bend the knee to allow it to swing through under the body. When starting from a stationary position Barr,Gleave and some other past top dancers say to start the movement first by bending the knee forward carrying the body weight with it. Moving backward, the foot of the free leg will move back as the knee of the standing foot is pushed forward. Standing still feet apart at full stride with weight centered between both feet, you should be able to shift your weight back and forth from foot to foot with EASE.