When you strike anything, be it a talent or approach you have, you can't use them for the rest of the investigation. For example, if you strike the "Become the labyrinth" talent, instead of just marking stress and gaining a clue when you hit the streets, you are forced to make a risk roll. During Downtime you can use your xp to clear those strikes before moving to another case.
Maybe slash would be a better word for it since are you supposed to make a marking in the sheet? I don't know. I will make it more clear in another version but I hope that helps for now!