The electron cyclotron maser (ECM) is an intriguing electrodynamical system characterized by nonlinear field contraction and other unconventional behavior. We report recent physics investigations of this class of radiation mechanisms, both in theory and in experiment [1,2]. Results indicate highly current-sensitive field profiles and hence sharply contrasting linear and saturated behavior. Transition from the stationary state to a sequence of nonstationary states in resonantor-based oscillations is experimentally characterized for the first time. By contrast, we have also demonstrated the stationary operation of backward-wave oscillations at a beam current far in excess of the generally predicted nonstationary threshold. This difference in nonlinear behavior is shown to be fundamental through a comparative analysis of the feedback mechanisms, energy deposition profiles, and field shaping processes involved in these two types of oscillations.