Imagine the world as a puzzle of 1000 white pieces.
When a Fuzetsu spell happens, the affected area is cut off from the flow of the outside world. Time, space, motion and existence itself are separated, and they are halted inside the Fuzetsu. The affected area still exists since people trapped inside can die and buildings can be destroyed, but from the perspective of humans outside of the spell, the affected area never existed.
To the supernatural entities of the verse, the puzzle becomes 999 white pieces and 1 black piece (the affected area), but the black piece is still attached to the puzzle, since it still exists.
To humans, they instead perceive the world as a completed 999-piece puzzle. The 1000th black piece doesn't register to them, because to them it doesn't exist. White the Fuzetsu is active, they don't even think about the black piece; the world always had 999 white pieces.
If we assume a Fuzetsu happens around the Eiffel Tower: while the spell is active, the tower still exists, but humans don't register it as "existing". In humans' perspective, it never existed. But when the spell is undone, the Eiffel Tower goes back to "having always existed" and humans don't recall the blank time where it had not.
Essentially, an area affected by a Fuzetsu is similar to if it had temporarily stopped existing from a human-world sense. While active, humans live as if it never existed. When undone, humans live as if it existed as normal. Same deal with people trapped inside the spell.
That's what I mean by saying the Fuzetsu does a Causality-based time stop + temporary existence erasure/memory manip/mind manip
I hope that's clearer.
