Original idea by: Flowerman
This technique is is called "slicing"
"I just take a 8inch blade with serrated edges, start at the top, about to 1 inch from the sides(depending on what stage they are in), and cut all the way down, cutting the roots, and I do all 4 sides this way, and sometimes the bottom. I know it sounds severe, and when I first started doing it, I would cringe at doing it, but I have been doing it this way everytime I transplant for the last few years, unless I'm using the pot within the pot method.
They have a little shock, but it only last for a day at the most, and then they just seem to just grow vigorously. Never had a problem with it.
It seems to promote new root growth on the sides growing lengthwise, and slows down the classic swirling that you usually get, which causes that classic rootbound problem."
Added by: Kunta wears a sarong
This technique is really best used when transplanting severely root bound plants, or for the long term, keeping clone mothers in the same size pot, or to help rejuvenate and regenerate plants after you have harvested them ( in addition 24 hours light , N etc )
In tests this year I found that plants that had the "root slicing" technique, plants re-vegged slightly faster than the plants that were left in the same pots and fertilized.