how does "Decreases negative effect on HP and STA" from equipment or mod work?
It protects your max stat from being reduced. So, if you had 100 STA and a decrease-negative effect on STA by 9, it means that if you had injuries, you would have 9 STA of headroom before your STA actually starts reducing.
There is an exception to this decreases-negative effect on DEF. For that, it increases minimum defense.