If you have 10000 tech points, the Commie bonus means it is calculated *as though* you have 12000 tech points. That is, you will buy 20% less tech points for the same tech% as other countries of the same acreage.

However, if you think about it, this is a somewhat weak bonus. When you sell your military units, you lose 10% to market commission, and when you buy tech with that money, you lose another 10% to commission. Buying 20% less tech points simply negates that disadvantage, for the portion of units and money you spend on tech points.