I feel like just one more slide on the tutorial saying "You lose health over time at an increasing rate" would go a long way to explaining this, and players could work out the interaction with health regen themselves if you don't want a long tutorial
Yeah, I must agree with you here. It might be as simple as letting players know that "you lose health over time". Brainfog Games , I actually did play the tutorial (one of the rare times hahaha) and i understood that i took damage when bopping on enemies, but I didn't quite get the "time" aspect of loosing hp, until a lot lot later.