Help with root pruning

i saw a picture in this link http://usanthony.blogspot.com/2009/01/growing-marijuana-managing-small-plants.html#comment-form and i wanted to do something like that just for fun. theres this thing called griffins spin out, but its only available in large pots. does anyone out there know how to prune the roots of a plant?

Just chop them with a sharp knife, like in these pics:

Pic 1
Pic 2

Taken from OT1's guide to bonsai mother plants here.

thanks a lot thats helpful!