Originally Posted by
Woreczko
As far, as i tested it, "impetous" unit is the same as "normal" one, except, that it`s morale in-battle may rise as high as impetous state (as opposed to a max of "eager" for other units). This is somewhat independent from "morale" attribute and results rather from in-battle factors, such as general nearby, winning battle, outnumbering the enemy in the area, having it`s flanks covered, etc... Even a unit with a very low default morale may enter impetous state. However under unfriendly cicumstances, it will break just as fast, as a normal unit.
Disciplined units do seem to have various in-battle morale penalties lessened.