Pasting info from reddit (more info and screenshots in the original post: http://www.reddit.com/r/CrusaderKings/comments/2vgehp/found_a_gamebreaking_exploit_for_ck2/).
Which basically means if instead of merging the units in one army you leave them as separate units, you'll get the battle bonuses added up for each little unit.When you win a battle, you gain Battle Score, wich adds up to your war score, Prestige, sometimes Piety (when in Holy War) and then Military Tech. Most of the time you get 0.5 military tech point at most, but in big battles it can go up to 15.
Now why is it a gamebreaking bug ? The number is calculated on the number of armies involved.
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