Board Thread:Questions and Answers/@comment-64.233.173.35-20160504014407/@comment-24356739-20160504081123

The 1HP protection is only when you get heavy damage when your sword is still in medium injury.

Eg: Entering battlefield with sword in medium injury 16HP > gets r5 yari twice (16) > sword HP drops to 1

This is what I think happened, although I can't guarantee it as I haven't tested out or observe this in details before.