In the framework of the structural instability studies in the archetype perovskite compounds, the unusual mechanism of phase transition occurring in potassium calcium fluoride , due to both condensation of R(0.5, 0.5, 0.5) and M(0.5, 0.5, 0) points of the first cubic Brillouin zone, was investigated by inelastic neutron scattering. The appearance of a central component, and evidence of the softening of the whole phonon branch (0.5, 0.5, ) of the reciprocal space, is displayed and discussed in the framework of theoretical models, which take into account the strong anisotropy of the coupling constants driving the in-plane or the plane-to-plane octahedron rotations around the [001] cubic axis. Furthermore the behaviour of the soft modes, responsible for the transition versus temperature, seems to indicate that an intermediate disordered phase with cubic average symmetry can be introduced in such systems. The basis of an original model describing this structural phase transition, where rotations of rigid octahedra are symbolized by pseudo-spins, is proposed.