The "switch function" is rigorously derived from first principles and is shown to be a fundamental and essential feature of any constraint release model for Doi-Edwards-type molecular models that invoke the concept of a discrete slip-link tube with separate descriptions of orientation and stretch. The switch function self-consistently apportions the fraction of Kuhn bond orientation relaxation attributed to tube stretch and orientation such that the same net fraction of Kuhn bond orientational relaxation per constraint release event occurs independent of the level of tube stretch, provided the chain obeys Gaussian statistics. The switch function is derived for the particular case of the Mead-Larson-Doi (MLD) model, with constraint release-driven tube shortening quantified by 1/2(lambda - 1). The MLD switch function is generalized to account for arbitrary non-Gaussian, levels of finite tube stretch. The MLD model tube-shortening process is also impacted by non-Gaussian, finite extensibility, effects and is generalized utilizing a second-moment Kuhn-Grun analysis. It is shown by performing a fourth-moment analysis of the Kuhn bond orientation that the same switch function derived by analyzing the second moment also applies to the fourth moment in the small stretch Gaussian limit.