Catchability
Outfielders "track" fly balls and position themselves to catch them by "zeroing out" the ball's vertical acceleration in the optical array.
Michaels, C.F., & Oudejans, R.R. (1992). The optics and actions of catching fly balls: Zeroing out optical acceleration. Ecological Psychology, 4, 199-222.
When the ball's vertical acceleration is negative, they know to move toward it.
Vertical acceleration is negative either when the ball is slowing in the optical array on the way up or speeding up on the way down.
When the ball's vertical acceleration is positive, they know to move back.
Vertical acceleration is positive either when the ball is speeding up in the optical array as it rises or slowing on its way down.
Another study suggests that outfielders determine "catchability" more accurately if they are able to move even as they judge the sign (negative vs. positive) of optical acceleration.
Oudejans, R.R., Michaels, C.F., Bakker, F.C., & Dolne, M.A. (1996). The relevance of action in perceiving affordances: Perception of catchableness of fly balls. Journal of Experimental Psychology: Human Perception & Performance, 22, 879-91.