Remember that the first move action is free, so feasibly a berserk model (let's just say Sven) could activate while exhausted per the berserk rule and move once, free of charge.
In order to do anything further that requires you to fatigue (an extra move, an attack), once per turn you may pay 1 favor, and instantly do damage to him until he gets to his next favor-generating damage box, instead of gaining a fatigue. Essentially, you take damage instead of gaining a fatigue as the cost for the action.
Does that make sense?
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