It’s a neurological movement pattern that’s hard-wired into us as biological beings, which causes a specific group of muscles to contract in response to a perceived sensation. The sensation forces a particular group of muscles to contract. For example, if you turn a baby’s head to the side, its arm involuntarily goes straight, and its leg goes straight on the same side, and the other arm bends. If this is happening after six months old that is a bit of a problem.
There are many of these, and they exist to teach us how to move our muscles once the muscle formed in the womb. They then help us through the birth process and help us move in the air after being in water for nine months. They get us into an upright position for walking and running from a curled ball when we are born.