What ever we want to achieve with our body, intent does the trick. Of course, this applies just to a perfect world, which does not and will never exist. However, our nervous system does an incredible job in refining motor coordination among various muscle groups to achieve possible goals. Fresh humans see bipeds walking and standing on their legs, and manage to copy this skill just by observation and experimenting with their nervous system.
Although most humans acquire their most important skills, moving their bodies at will in the manner of bipeds and communicating, without any guided teaching, kindergarten and school replace the experimental learning of the nervous system with structured exercises strengthening our logical abilities.
The curiosity driven, holistic approach to learn about one's surroundings get channeled into other pathways, and the combination of bad examples and detrimental cultural habits form the basis for a slowly growing mind body disassociation. We learn to use the mind to control our body, and contort it according to fashion and furniture. Our brain wants to keep the body in balance - a released neck allows plenty of air to enter the lungs, and refreshed, oxygen rich blood can flow through the brain. But as we copy our family's ideal of comfortable sitting we use conscious involvement to override the brain's control of the body.
After a few repetitions of consciously interfering with our motion control this action becomes a habit, available to fulfill our desire for comfortable sitting, independent of the feedback our body gives. The commands Sit still! and Sit straight! come without further explanation, and yield a mostly random bodily response.