It is based to constitution..
.if you have a 19 in con and suffer a blow of 10hp you don't get any damage
To get affected you must endure 20+ damage
If so, you drop one level of Constitution
If you took 40 HP damage you would drop one level (which put you to con 19 ) and a second level === >. Con 18
Excessive damage ( in this example 19+18=37 out of 40 ) are lost
Have you ever played Star Wars Saga?
Easily one of the best, and most innovative, D20 systems I've every played, or DMed. Here's a breakdown, and how it's similar to what you're showing:
Hit Points: just like D&D
Constitution: sounds familiar?
Damage Threshold: what you just described
Damage Track: eerily similar to the 5e Exhaustion Track
Damage Threshold is based on your Con score, Class, Class Features, and Armor. In Saga, the heavier the armor the greater your damage threshold. Anyway, let's break down the mechanics.
Storm Troopers shoot you for 3d8 damage (everything has a high dice damage in Saga), hitting you twice!
Here's the breakdown of the character:
- Con score of 11 (that's base Threshold)
- Add half of your heroic level rounded down. As a 3rd level character you get +1.
- Wearing scout armor which provides +4 Damage Threshold (16 total)
- Let's say you have 41 hit points.
The first roll is very high 17! The second is average damage for 13pts. When your Damage Threshold is surpassed you move down the damage track. The damage track has a penalty to all of your rolls and your Threshold. The first attack would break your Damage Threshold which moves you down the Damage Track one. Even with the damage your second roll would barely miss your Damage Threshold (15 would drop down to 14 for moving down the Damage Track). You have 11 hit points left and with your scout armor the average damage won't break your Threshold, meaning you're merely KO'd rather than killed.
Healing was difficult and slow if it ever passed your Damage Threshold. Hit points came back rather easily. Everyone had a Second Wind where they could regain hit points with an action. You get one per long rest. The explanation was like getting shot but your bullet proof vest absorbs it. You take non-lethal damage (don't move down the track because the armor increased your Threshold), but you get the wind knocked out of you and badly bruised. You get some back with your Second Wind but the rest of the hit points will take some time.
