I must change my early statement that ther is an aditional curving of the body at the end of sway. Having looked more carefully at the vidios I see that the sway starts very early on the first step(right foot) by staying low (not rising to soon) and make sure that the left foot extends to it's full length the top part of the body is left behind giving the sway,most of the sway is completed befor the weight moves on to the left foot. (The lady can help with the sway by stepping under her body with the left foot,this is my idea and may be bad advice for the lady.)