The "W" sitting position is one of many sitting positions that most children move into and out of while playing. The problem with this position is it limits active trunk control and development of the trunk muscles for independent sitting. If the child prefers to sit in this manner, it may be difficult to change the habit due to decreased strength in the trunk. Excessive use of this position during the growing years can actually lead to future orthopedic problems.
Why do children W-sit? Balance! Children who are frequent W-sitters often rely on this position for added trunk and hip stability for easier toy manipulation and play. When sitting in this position they are planted in place through the trunk which allows for frontal motion but does not permit trunk rotation and lateral weight distribution (twisting and turning to reach toys on either side). Trunk rotation and weight shifts allow a child to maintain balance while running outside or playing on the playground and are necessary for crossing the midline while writing and doing table top activities.
How to stop W-sitting? To prevent W-sitting is to prevent anticipate and catch it before the child even learns to W-sit. Children should be placed and taught to assume alternative sitting positions. If a child discovers W-sitting anyway, help him to move to another sitting position first passively and then using toys or other aids as motivators to encourage alternative sitting positions like the "z stagger" or sitting with legs forward.