I only just recently started talking about this.
During a short rest, one PC regains hit points. A short rest must be called for by a PC who’s bloodied or whose death spiral has moved beyond “Normal.”
I’ve been trying to come up with a method to discourage resting in the dungeon, and previously I’d come up with exhaustion/exposure mechanics that dealt damage based on when and how the party rested. This might be better.
Exposure worked on the premise that adventuring was hard on a body, that it caused stresses that would build up — resulting in psychic damage (which would scale by tier) at the end of any extended rest taken during an adventure.
Exhaustion worked on the same basic premise, but reflected stresses put on a body throughout an adventuring day. Each PC would take necrotic damage at the end of any short rest taken during the day.
Both mechanics have the same problem that GMs have to remember them, and the damage might be variable — if the party is resting to recover hit points, what’s the big idea doing damage to them immediately after resting?
Instead of exposure and exhaustion being an active thing that players must remember to use during play, we can assume that they’re a constant passive effect — which work together to prevent the PCs from recovering.
There, we have our explanation for why PCs don’t all “heal-up” during a rest — for the most part, the timing is wrong. The situation is too stressful. Only one PC can benefit from resting at a time — the others must stay alert and stand watch.
It’s important to note how this is different from 4e Healing Surges or 5e Hit Dice — both of these mechanics provide a limited quantity of healing on an individual basis, and there’s literally no benefit for a character resting who has run out.
During a short rest, any one character will recover all current damage, or will move a step back up the death spiral. No additional mechanics required.
Additionally, resting will provoke a check from the current mission — I haven’t worked out terminology (I like “urgency”) or effects yet — resting bears with it the consequence that stuff might happen without the PCs.
Now, with concepts like exhaustion and exposure covered by “only one PC rests at a time” the question becomes one of how to modify and enhance downtime with mechanics like “trust” and “intimacy” from Bliss Stage.