It flips life. For Triphoon, it only flips its own life.
So… 1/1000 turns into 999/1000.
500/1000 turns into 500/1000.
1000/1000 turns into 1/1000.
10/1000 turns into 990/1000.
300/1000 turns into 700/1000.
501/1000 turns into 499/1000.
You get the idea. Basically you take your HP left and subtract it from your max HP and that’s the result.
So it has the highest positive effect if used when you have a very low amount of HP and does the highest damage if you use it when you have a lot of HP. Provided your HP is below half, it will benefit you.