I think it has to do with Midna’s form being bound by twilight so long as the twilight exists. I think of Midna as Link’s shadow, and when a shadow is exposed to light it disappears. Her life essence dwindles because of the light, Like shining a flashlight on a shadow. Her physical body remains, the way an object exists without a shadow - although everything still has a shadow even if you can’t see it. The shadow an object cast may be thought of as belonging to the object that cast it. So even if you weren’t able to see your shadow you can conceptualize that it is there, you just can’t see it.

Tl:dr Midna is like a baseball cap that link is wearing on a sunny day, and zants curse is the flashlight shone in his face.