Multiple-resonance thermally activated delayed fluorescence (MR-TADF) emitters are becoming increasingly attractive due to their applications in high-resolution organic light-emitting diode (OLED) display technology. Here, we present an investigation on the photophysics of two MR-TADF emitters (t-DABNA and TBN-TPA) by using quantum chamical calculation and ultrafast transient absorption (TA) spectroscopy. Compared with one-step structural planarization of t-DABNA, TBN-TPA undergoes two-step relaxation in S1 state, i.e., fast twisting of the peripheral group and subsequent restrained planarization of the B-N framework. The efficient twisting motion of the peripheral group largely reduces the energy level of the TBN-TPA system and correspondingly increases the barrier for subsequent planarization, which is favored for the narrowband emission. Our work provides a detailed picture for the excited-state deactivation of peripheral group-modified MR-TADF emitters without a pronounced charge-transfer (CT) characteristic mixed in the lowest-lying fluorescent state, which might be helpful for the future design of narrowband OLED emitters.