Two-channel and multichannel thermal unimolecular reactions are analyzed by simple models, starting with the calculation of separated-channel rate constants and accounting for intrinsic channel coupling afterward. Reactions with rigid- and with loose-activated complex channels are distinguished. Weak-collision, energy-transfer, effects are suggested to govern the competition between rigid-activated complex channels, while angular-momentum, "rotational channel switching", effects dominate the competition between rigid- and loose-activated complex channels. The models are tested against master equation treatments of the dissociations of formaldehyde and of glyoxal from the literature. Besides giving insight into the influence of various molecular input parameters, the present approach leads to compact representations of rate constants suitable for inclusion in databases.