  We provide a comprehensive analytical description of the effective coupling
associated with an antiresonance within a hybrid system comprised of a
quasi-closed photonic cavity and a ferrimagnetic material. Whilst so-called
level attraction between a resonant system inside an open cavity is well
understood, the physical underpinnings of this phenomena within quasi-closed
cavities have remained elusive. Leveraging the input-output theory, we
successfully differentiate between the repulsive and attractive aspects of this
coupling. Our proposed model demonstrates that by understanding the phase-jump
at the resonances and the studied antiresonance, we can predict the nature of
the effective coupling of the antiresonance for a given position of the
ferrimagnet in the cavity.
