It should be listed as a status effect on the left - next to your character's portrait. It activates when you are below 30% HP and gives +2 on attack rolls and wisdom ST.
Yes, it's a bit weird. I think it's a Condition (rather than a Passive Feature) that as far as I can tell lasts until your character dies - totally dies that is, needing resurrection, rather than simply being knocked out. I hadn't realised this until I suddenly noticed the condition had vanished and did some experimenting to find out why.
If it's going to last that long, you might as well make it a permanent Passive Feature that appears on the character sheet. Though arguably it's a bit of an overpowered boost for how relatively easily you get it, which can discourage RP, in which case its duration should perhaps be reduced, perhaps just until the first time it's used or until Long Rest.
I got it for my paladin playthrough (trying to see if it would break my oath - no!) but hadn't done it for ages before that. I didn't think it used to last that long, but maybe am misremembering. Or maybe my paladin playthrough had a bug!
