Aggregation-induced emission (AIE) phenomenon has attracted much attention in recent years due to its potential applications in optoelectronic devices, fluorescence sensors, and biological probes. Restriction of intramolecular rotation has been proposed as the cause of this unusual phenomenon. Rational design of AIE luminogens requires quantitative descriptions of its mechanism. 2,3-dicyano-5,6-diphenylpyrazine (DCDPP) with "free" phenyl rings is an AIE active compound, whereas 2,3-dicyanopyrazino [5,6-9,10] phenanthrene (DCPP) with "locked" phenyl rings is not. Quantum chemistry calculations coupled with our thermal vibration correlation function formalism for the radiative and non-radiative decay rates reveal that the radiative decay rates for both DCPP and DCDPP are close to each other for all the temperatures, but the non-radiative decay processes are very different. For DCDPP, the low-frequency modes originated from the phenyl ring twisting motions are strongly coupled with the electronic excited state, which dissipate the electronic excitation energy through mode-mixing (Duschinsky rotation effect), and the non-radiative decay rate strongly increases with temperature. For DCPP, however, such mode-mixing effect is weak and the non-radiative decay rate is insensitive to temperature. These findings rationalize the fact that DCDPP is AIE active but DCPP is not, and are instructive to further development of AIE luminogens.