Putting Hammered or Crooked on a tool that has the Unbreakable trait makes it take damage when used for the "Hammered" or "Crooked" purpose. For example, a pick with Hammered will take damage on anything hammerable, but not on say, marble, which isn't hammerable. Also, a hatchet with Crooked will not take damage mining wood, but will when breaking leaves, where it wouldn't have taken damage breaking leaves before applying the modifier.