so i was testing Gemini Magatama a while ago, i used Water Dragon Spear Rank V,
and this is what i found out.


in my first attempt, my hp is full. DMG = 4615


in magatama description it says, " if remaining HP: 50% or more, Rank 4+ Spear Skill Force 39%.
so i lowered my HP to below 50% in my 2nd attempt. DMG = 4623


just to be sure i decided to try again, this time my HP is Above 50%
coz i was skeptical about " HP: 50% or more " maybe it means 50% " above " and not " Below "
the result. DMG = 4631


is Gemini working? or just i dont know how to activate it?

Try testing with it off.

as SupremeTentacle suggest, i tried it w/o equiping Gemini Magatama.
and here are the results:







conclusion:
the condition " if Remaining HP:50% or more " is Null.
at least for Gemini Magatama, i dont know if that also applies on other Magatamas.

PS: i had a hard time taking screenshots of the atk dmgs. so please bear with it. i know its nearly not visible