Sit flying is a form of locomotion obtainable to skydivers and vertical wind tunnel flyers. It describes the ability to 'fly' ones body in any direction while in a 'sit' position. It's called a 'sit' because it looks like the flyer is sitting in a chair.
The correct position for flying a sit is to orient yourself feet first in the direction of the relative wind and maintain 90-degree angle bends at the knees, hips, and shoulders. Wind from the tunnel or from free-fall should be hitting the flyer on the feet, hamstrings, bottom, and the underside of the arms. To move about relative to the tunnel or other jumpers, the flyer simply redirects the airflow the opposite direction the jumper wants to go. Newtonian mechanics then push the flyer in the preferred direction. Fall rate changes (descending faster or slower) can also be made. This is done by presenting more surface area (wide legs and arms, higher drag, slower fall rate) or less surface area (arms up or even stand up, less drag, faster fall rate) from the flyers body to the wind.