Pulsational Pairt instability is when a vertex is formed of perturbative scattering matrix between sufficiently high frequency photons propagates electron-positron pairs. As such, there is a pressure change as the particles attract each other both electromagnetically and in principle, gravitationally - scaled up to the number of events within the core and this sudden collapse is a mammoth 'hiccough' as the outer layers are expelled at a phenomenal rate.