In order to understand the fundamentals behind pulsed plasma thruster discharge initiation, the phenomenon of undervoltage (i.e. at a voltage slightly below the breakdown voltage) breakdown through electron pulse injection is explored. This phenomenon is not only the mechanism employed by sparkplug-based PPT initiation systems, but is also the basis for a new optical initiation system which promises improved performance and lifetime. A theoretical model is derived which predicts the injected electron density required to induce breakdown. These results are compared to experimental measurements in which laser pulses on a tungsten surface are used to cause electrons to be injected into a discharge gap with a parallel-plate geometry. It is found that for argon at 2 Torr, the theory and experiment both give the required current density on the order of 10-7-10-8 A/m2 at voltages ranging from 90% to 99% of the breakdown voltage. The similarity suggests that the theoretical interpretation is reasonable: the pulse of electrons alters the space charge in the gap and augments the electric field, making ionization more likely and causing the gas to breakdown.