I think that the points get evenly (or randomly?) distributed among all unresearched upgrades that you have the capacity to research.
It is research points redistributed "randomly" (seems random anyways) in the certain tree, if you trade for magic knowledge, it is research points redistributed in the magic tree.
This is to replace a tech trading system, and I am happy they got this idea, because when you know how it works its quite interesting.
Try midgame to check out your warfare tree, then trade for 200-300 warfare knowledge and check it again, to see the processes of the techs advance.
This will only apply to techs that you can currently start researching btw.