‘W’ sitting is when a child sits with his bottom on the floor, the hips spread, knees bent, and his feet behind him, making a “W” shape with his legs. (Fig. 1)
Fig. 1. Child sitting in the ‘W’ position
Tenodesis refers to surgical fixation of a tendon. This fixation involves transfer of a tendon from its initial point of origin to a new origin to restore muscle balance to a joint, to restore lost function or to increase active power of joint motion. (It also involves closing of the fingers through tendon action of the extrinsic finger flexor). 
Yes, there does appear to be a ‘functional tenodesis effect in trunk stabilization’ as patients often compensate for the lack of muscle activity or other motor deficits by using functional means such as W sitting observed in children with poor trunk stability or control. There has been recent debate in the scientific community over the perceived harmful effects of W sitting – with some clinicians actually making a case for the benefits of W sitting (in other words, that W sitting isn’t harmful at all or that the perceived harm of W sitting is not supported by evidence).
Nevertheless, here are the pros and cons as presented by their respective proponents. Let’s start with the cons:
- Orthopedic concerns. W sitting can predispose a child to hip dislocation, so if there is a history of hip dysplasia, or a concern has been raised in the past, this position should be avoided.
- W sitting tends to aggravate muscle tightness where present. This position places the hamstrings, hip adductors, internal rotators and heel cords in an extremely shortened range. If a child is prone to tightness or contractures, encourage anther pattern of sitting.
- Neurologic concerns/developmental delays. If a child has increased muscle tone (hypertonia, spasticity), W sitting will feed into the abnormal patterns of movement trying to be avoided. Using other sitting postures will aid in the development of more desirable movement patterns.
- W sitting can also discourage a child from developing a hand preference. Due to an absence of trunk rotation when W sitting, a child is less inclined to reach across the body and instead picks up objects on the right with the right hand, and those placed to the left with the left hand. Trunk control and rotation necessary for midline crossing (reaching across the body) and separation of the two sides of the body. These skills are needed for a child to develop refined motor skills and hand dominance.
- Children often rely on this position for added trunk and hip stability to allow easier toy manipulation and play. In the W-position, a child is planted in place or ‘fixed’ through the trunk giving them a wide base of support which makes them more stable. It therefore appears that for many affected children, W sitting isn’t a problem, it’s their solution.
Regrettably, I am unable to provide a suitable text-book reference.