It never triggers. I have over 6000 health and it triggered for me twice when I was around 100 health. Some Administrator said here on forum years ago that it triggers more often around 5-10% of max health, and it does not seem like it does. You can go on unicorn lane as max wizard and they ghost will kill you before it triggers and its considered to be an epic talent. It is either bugged or needs to be changed, it also looks really old while it happens, doesn't look like something that can be so epic as saving life should look. The card revitalize looks on point though, and the spell while it happens should look how the card does.
