Parabolic motion
From Footbag Reference
Parabolic motion is a term that describes the motion of the footbag through the air. Often the bag is described only as moving up and down, which is just for the sake of simplicity.
Not only does the bag move towards, away, to the left, and to right(perspectively) of the player, the player will often set the bag into a parabolic motion that allows him/her to hit the trick more easily.
Parabolic motion can be easily noticed in consecutive far butterflies, which are also called infinities, because the motion of the bag is in that of an infinity symbol.