As a reaction, which you use when you make a Fortitude saving throw, you can focus your mind to overcome physical pain and control your body’s response. Roll a d3 and add the result to the roll. You can do so after the roll but before any effects of the roll occur.
You can expend 2 additional stamina dice to roll 2d3.
You can expend 4 additional stamina dice to roll 3d3.