When another player narrates a PC's action, such as "I attack the ogre", you can ask "what if...?" You must propose an interesting alternative outcome, such as "what if the ogre smashes you across the room?"
If anyone else in the group accepts the What If, and no player vetoes it, the original player must get a Hit for their narration to prevail. They can also opt to accept the What If.
What Ifs can pre-empt the action ("your attack doesn't land, the ogre smashes you") or add to it ("your attack lands, and the ogre counters").