3: Powered By Fate
From p. 160 of Fate Core
One of your options to mitigate the effect of a hit is to take stress.
The best way to understand stress is that it represents all the various reasons why you just barely avoid taking the full force of an attack. Maybe you twist away from the blow just right, or it looks bad but is really just a flesh wound, or you exhaust yourself diving out of the way at the last second.
Mentally, stress could mean that you just barely manage to mitigate some comment, or clamp down on an instinctive emotional reaction, or something like that.
Stress boxes also represent a loss of momentum – you only have so many last-second saves in you before you’ve got to face the music.
On your character sheet, you have a number of stress boxes, each with a different shift value. By default, all characters get a 1-point and a 2-point box. You may get additional, higher-value stress boxes depending on some of your skills (usually Physique and Will).
When you take stress, check off a stress box with a value equal to the shift value of the hit. If that box is already checked, check off a higher value box. If there is no higher available box, and you can’t take any consequences, you’re taken out of the conflict.
You can only check off one stress box per hit.
Remember that you have two sets of stress boxes! One of these is for physical stress, the other for mental; you’ll start with a 1-shift and a 2-shift box in each of these. If you take stress from a physical source, you check off a physical stress box. If it’s a mental hit, check off a mental stress box.
After a conflict, when you get a minute to breathe, any stress boxes you checked off become available for your use again.