Do you happen to know if it's in the rules/FAQ somewhere?
Page 19 of the rules:
Modifier tokens trigger a number of in-game effects. They can, for example, force you to use an additional pawn, or to reroll a success die. All the icons on the Modifier tokens are explained in the table at the end of the rulebook.
Whenever an effect introduces a new Modifier token into play, you will be instructed where to put this token. Modifier tokens do not act retroactively — they cannot influence an action that was already declared even if it hasn’t been resolved yet. They do apply, however, to all future actions until the end of the round in which they’ve been discarded.