can anyone explain how it works?

also is commie/techer feasible in FFA?

If you have 100000 units of tech, the game treats it as though you really have 120000 units of tech. If you have A LOT of tech so that you are near the tech% limits, this bonus is essentially near useless due to diminishing returns on each additional unit of tech.

Also a commie techer is not feasible on any server, because of the 10% commision (you lose ~20% of your income just by selling tech and buying everything else you need (food, military, oil)), and because you aren't leveraging on the +35% commie military production bonus.

Commie indy is the only strat that works for commies, because the +35% production more than negates the 10% commision, and the +20% tech bonus cancels out having to pay 20% extra when selling military to buy tech.