According to the University of Washington (no affiliation - first Google result) "Air pruning happens naturally when roots are exposed to air in the absence of high humidity. The roots are effectively “burned” off, causing the plant to constantly produce new and healthy branching roots."
What this effectively means is that you don't get the "death spiral" of roots that restrict plant growth, instead, new roots are constantly forming to effectively grab more nutrients for optimal plant growth.
So Let's get on with it now the science lesson is out of the way :)
I like the concept of Air Pruning but don't see spending 4-5 times the money on a special pot so I elected to make one myself (thanks to some ideas from friends on this Facebook Group)
First things I needed were a pot and a donor plant. A Satsuma tree that my oldest son mishandled and broke because he did not want to bring it in the garage when temps dropped last week was a perfect subject...
So what do you need?
Pot, drill (to drill holes) and 2" (ish) hole bit, potting mix, landscape fabric, - you know...the usual stuff.
Hover over the images from here as it is pretty straightforward. Also, do me a favor...if you like this instructable, click that little link at the top of the page that says "Vote" and help me out in this contest :)