You step in to face a consequence that one of your teammates would otherwise face. You suffer it instead of them. You may roll to resist it as normal. Describe how you intervene.

This is how you do the "I'll dive in front of the bullet" You cover for a teammate, suffering any harm or consequences that still linger after you've rolled to resist. It hurts, cost stress, and may leave you in a bad spot. But hey, you're a hero.