It works when you fortify to heal, as other have said.
It's a very strong card imo - I used it in my last Byzantium domination game almost exclusively apart from in the first years in the Classical GA (monumentality and exodus), only slotting in the occasional Exodus if I needed some fast missionaries and could reshuffle my cards with a new civic a few turns later.
Though even then, Praetorian Guard stayed on almost exclusively, since when you're constantly pushing, +10 healing per turn is making your sieging units take almost no damage.
About time that warring civs get a good card as well imo, it was frustrating to have no good choices earlier on and it's one reason I'll probably stick with Dramatic Ages for a while.
Too often I've found that a golden Age really only helps me loyalty-wise, but otherwise not directly in combat.