Exert

Though you are weary from combat and toil you push forward, your mind ruling over the tired muscles. You inhale deeply, clench your teeth and brace against the air within your lungs...

You gain 3 exertion charges. Each time you exert an attack or yourself you spend an exertion charge. You replenish all charges with a short rest.

Exerting attacks

You can exert attacks. After you hit an enemy you can choose to exert the attack adding the maximum roll of a single hit die for your weapon on top of the damage you would otherwise have dealt. Example: +12 damage for a hit with a greataxe. Choosing to exert a critical hit makes it do maximum damage (maximum roll on all weapon dice)*.

Exerting yourself 

After failing a strength check or save you can choose to exert yourself replacing your roll with a 20, you then add your modifiers as if you rolled a 20. Example: A wall starts falling on you and you roll a 15 +1 strength modifier, thus failing the DC 18 to hold the wall. You can then choose to exert yourself making your roll 20 +1 strength modifier, allowing you to hold the wall and escape.

*This applies only to the hit dice of the base weapon! For example choosing to exert a critical hit with a greataxe would make do the maximal roll of its hit dice (2d12 on a critical hit), so 24. Exertion does not affect any other hit dice relating to the attack such as Sneak Attack or Divine Smite.