Warrior General Games

It says in the description that it helps catch up to speedy, agile ballcarriers. Does that mean it only triggers after a catch and not while you're covering a WR?

It means that any time you change direction you won't slow down as much as if you did not have this SA. I have not seen any reference to "only while covering a receiver". While it is possible the scenario you describe is the way it works, I doubt it. Typically the SAs that are situation specific have descriptions that are slightly more clear about this.

It's a good question considering how vague in general SA descriptions are but in this case I think I would assume it works any time you change direction. I haven't tried it myself to give you a clear answer.

Change direction is on the tree for many defensive positions. It is the defensive equivalent of quick cut for offensive players and works as you said, by reducing the speed lost while changing directions and allowing the player to cut at sharper angles.

I believe that it works when you are not covering WRs too, since it works for other positions that don't cover WRs at all.

Bort also added that if change directions is high enough, you can change directions in ways that are aphysical.