https://www.nytimes.com/2020/06/29/science/flying-snakes-physics.html

A professor of biomedical engineering and mechanics at Virginia
Tech, Dr. Socha and his colleagues published a study on Monday in Nature Physics
supporting the hypothesis that the midair undulations (the wiggles) are actually
carefully coordinated and highly functional processes that enhance the dynamic
stability of the snake in flight.

Flying is a bit of a misnomer for what the snakes do. The slithering airborne
creatures tend to fall strategically or glide, meaning they do not gain altitude
like a bird or an insect. Their flights generally last only a couple of seconds,
at a speed of around 25 miles per hour, and they land without injury. To the
untrained eye, it might look as if the snake just fell out of a tree by
accident, wiggling frantically as it plummets to earth. Not so.

Once it goes airborne — after inching out on a tree limb and pushing off the
branch — the snake moves its ribs and muscles to extend the width of its
underside, transforming its body into a structure that redirects airflow like a
parachute or a wing. A cross section of the snake’s body midair would show that
its normal circular shape becomes triangular and the whole body undulates as it
glides toward its target.