Airbrushing is when you hit the outside rim of the disc to keep it in flight. By performing this trick under your leg, you are adding a restriction which increases the difficulty of the trick.
Since the disc, you leg, and your hand are all moving, an under the leg airbrush can be performed at least three different ways…extra credit for listing another way in the comments.
First is where your arm reaches under your leg. The disc never actually traverses under your leg but your arm movement is restricted by your leg.
Second, the disc traverses under your leg before you airbrush it. The restriction here is the timing of the leg and hand movements so the disc does not contact your leg and so you do not air brush your leg instead of the disc.
Third, you brush the disc so it travels under your leg after the airbrush. The restriction is that the airbrush must be accurate enough and/or your leg must move accurately enough so the disc does not hit your leg.
Can you see the differences in the video? Which one do you think is the most difficult? Can you think of another way to do an under the leg airbrush with the same leg/hand combination?