Cryogenic plasma etching is a pivotal technology for next-generation 3D semiconductor devices with high-aspect-ratio and nanoscale features. During reactive ion etching of SiN in hydrogen fluoride (HF)-containing plasmas, an ammonium fluorosilicate (AFS) layer is formed; however, the precise role of the AFS in etching kinetic has remained highly controversial since its discovery in the 1980s. This lack of consensus has left the SiN plasma etching mechanism largely obscure, even within the context of modern cryogenic processes. Here, we elucidated this mechanism using in situ spectroscopic ellipsometry and attenuated-total-reflection FTIR while manipulating substrate temperature (Ts), bias voltage, and HF partial pressure via Ar dilution. We reveal that the etch rate (ER) and the AFS thickness are established through a dynamic balance between HF-induced AFS formation and simultaneous removal by ion bombardment. This interplay results in a characteristic bell-shaped ER dependence on Ts and Ar content. Crucially, whether AFS promotes or retards etching depends on the rate-determining step: in removal-limited regimes, ER is inversely proportional to the AFS thickness, whereas in formation-limited regimes, they are positively correlated. The proposed model suggests that ER at low Ts can be significantly enhanced through high bias voltages or synergistic chemistries that catalyze AFS depletion.