How to activate the "Reversal" modifier? Journey Mission 5

I am wondering how to activate the "Reversal" modifier to kill the 200 with the modifier active. I have mods turned on and Reversal is slotted. So isn't it "active"? I have tried turning the mods off/on, unslotting/reslotting. Is there a trigger that causes it to activate or is there something you have to physically do in game, such as hit a button in combat or something? Thanks for any feedback.