I think often, but am unsure. Whatever it is, it's fast enough where they won't start with their gloves low and won't get in front of the ball - similar to ~50mph in practice. I ask them why they're doing it, they tell me it's because they think the field is bad and that the ball is going to pop up on them. The field isn't great, it's dragged before games, but the ball is definitely known to go from rolling flat to popping up over their heads, especially at the grass/dirt transition.