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Megavolt-class pulsed gas switches play an important role in large pulsers as transfer switches. This article analyzes the breakdown process and characteristics of a megavolt self-triggered preionized switch under 100-ns pulses based on the optical diagnosis. It is discovered that the triggering spark illumination targeting the main cathode or anode can separately preionize the main gap. The discharge channel is likely to first form between trigger electrodes and the main anode. Based on the above deduction, the streamer velocity is calculated to be $0.54 \times 10^{6}$ – $2.34 \times 10^{6}$ m/s, proportional to the electric field. Also, parameters of the empirical formula for calculating the gas heating time are amended. Influence mechanisms of the switch jitter are also studied. Under pulsed preionization mode, the trigger gap breaks down when the main gap electric field is high enough, and the switch jitter is mainly determined by the self-triggering jitter. Nonetheless, the self-triggering jitter can be partly offset during the breakdown process if the changing rate of mean reduced electric field $d(E/p)$ / dt at the self-triggering time is higher than 1.0 $1.0\left(\mathrm{kV} \cdot \mathrm{ns}^{-1} \cdot \mathrm{cm}^{-1} \cdot \mathrm{MPa}^{-1}\right)$. Under sustaining preionization mode, the trigger gap breaks down prematurely, and the continuous arcing driven by switch leak current preionizes the main gap. Therefore, the influence of the self-triggering jitter on the switch jitter can be eliminated. [ABSTRACT FROM AUTHOR]