basically there are type1, and type11 fibres in all muscles
essentially type 1 ( slow twitch fibres) are found in postural muscles, calves, lower back etc
type 11 ( a , b and what ever else they want to call them) are the fast twitch fibres used for movement and max exertion.
you expend more energy by recruiting fast twitch fibres. The body on the whole wants to work as efficiently as possible, so in normal movement it is going to firstly recruit the slow twitch fibres. when you perform a maximal contraction the body moves through the stages of recruitment. It basically freaks out and goes " holy shit, the type 1's can't handle it, get in the reinforcements" so the type 11a come in, then type 11b etc
i guess this program is similar in theory to 5x5, where you smash 5x5 then do 2 x 12. I have trained 3x3 then hit some higher rep 2x8-12 and had significant size and strength gains from this without overtraining.
Overtraining would be a concern for me after doing a some heavy 3x3 stuff, folllowed by 3 x8, and then 3 x 20 ( shit that 3 x 20 would be a killer).
I am very interested to see how this training would work for you, so I hope you give this a serious shot and would encourage you to post your workouts in the journal section.