The present work examined the shock dynamics (i.e., speed, acceleration and front curvature) of detonations during sub-critical diffraction, where the complete quenching events eventually took place for the initially coupled shock-reaction complex. The compiled dynamics results showed that, while the ray-tube-based \(D(\kappa)\) relationships of detonations off the axis, constructed using our recent curved ray-tracking method, departed evidently from the previously established quasi-steady dynamics, those near the axis before failure appear to collapse excellently. Such agreement as well as the coincident propagation limits verified the validity of our previous quasi-steady curvature-based model in predicting the critical detonation diffraction. Furthermore, we quantified the ignition regime using a characteristic ignition criterion and demonstrated its coincidence with that of quasi-steady descriptions. Finally, we demonstrated the self-similarity of the quenched detonation shock front decay dynamics, which can be excellently captured by our weak shock support model.