  Strong interactions between different degrees of freedom lead to exotic
phases of matter with complex order parameters and emergent collective
excitations. Conventional techniques, such as scattering and transport, probe
the amplitudes of these excitations, but they are typically insensitive to
phase. Therefore, novel methods with phase sensitivity are required to
understand ground states with phase modulations and interactions that couple to
the phase of collective modes. Here, by performing phase-resolved coherent
phonon spectroscopy (CPS), we reveal a hidden spin-lattice coupling in a vdW
antiferromagnet FePS$_{3}$ that eluded other phase-insensitive conventional
probes, such as Raman and X-ray scattering. With comparative analysis and
analytical calculations, we directly show that the magnetic order in FePS$_{3}$
selectively couples to the trigonal distortions through partially filled
t$_{2g}$ orbitals. This magnetoelastic coupling is linear in magnetic order and
lattice parameters, rendering these distortions inaccessible to inelastic
scattering techniques. Our results not only capture the elusive spin-lattice
coupling in FePS$_3$, but also establish phase-resolved CPS as a tool to
investigate hidden interactions.
