Well it's been made out to me that because it feels the air and not a pot it grows back in on its self hence the term air pruning, and when it hits the bottom it curls back in for a bit to fill the rockwool cubes entire mass instead of the tap root going down and just stopping with a few roots branching off the main root system U get a whole ball inside the rockwool first then plant in coco...
There's a link he's an awesome grower