When can the use of an Absorb Blow charm be declared?

[utsukushi]

Huh. It was always my impression that you activated it, like, any time, whereupon they essentially add 12 to your health until it's used up.

So if you activate it on Tuesday morning and don't get into a fight until Friday, it's still there, actively ready to absorb damage. But if you haven't activated it, it's not doing anything. So it's not so much meant as, "I want to absorb this attack," as it is, "This battle is important enough that I want some extra health," or, "Today doesn't look like a good day to die, actually."