Just as the fencer reflex (ATNR) splits the body down the middle, hindering left-right coordination, this cat-sit reflex (STNR) splits the body across the middle, hindering top-bottom communication. As long as the STNR remains imperfectly integrated, a human being cannot attain fully upright posture -- because the reflex dictates that when the upper body is extended, the hips and knees want to flex.
In this youtube video clip a baby is giving a lovely demonstration of the STNR. In learning how to crawl forward, he actually pushes himself backwards. This is because when his neck extends his arms extend, and the extending arms push his body backwards. It nicely illustrates how a primitive reflex, as long as it remains immature, is liable to cause an unconscious reaction in the direction which is opposite to the one intended.