The main concept for no treatment of the bees is to only feed them honey
and natural pollen as well as to regress their size to 4.9 mm (now bees
are much bigger). It is a quite simple concept but needs some effort at
the beginning.
There are members that have not treated their hives with nothing (not
even natural substances i.e. EO) for decades.
I have no personal experience but i will be starting this spring
(March). I found out from a friend of mine who has about 300+ hives in
Canary islands.