You prepare to act on a condition. Choose another activity that requires either a short activity or a main activity. The action required to Ready a Response is equal to that of your prepared activity. Then choose a condition under which you will take the prepared activity. Hold 1.
When the chosen condition becomes true, you may spend a Hold to take the prepared activity as a free action, even outside your turn.
If your next turn starts, and you still have Holds from Readying a Response, those Holds are lost.