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
@ironnoob if your worried about lack of light stunting them just use 600w hps from day one. Keep it about 3foot above them for the first week then lower it.
Ive been doing that for years and havnt burned a seedling yet