My daughter has Down syndrome and low muscle tone. She sat in the W position also. A physical therapist can explain it better, but it has to do with weight distribution affecting how the hips develop. (The curvature of the socket part of the ball and socket joint, you want it curved and not flat or "loose"). The more they sit in the W position, or any other incorrect position, the harder it is to develop the hips and muscles correctly. So they just have to be taught the correct positioning, which a physical therapist can do. My daughter recieved therapy through Early Intervention, but there are also great therapists at Kennedy Krieger.