SPHLC/Nutloose
The handle operates a mechanical linkage that pulls a sear out of the firing unit. There will be a small amount of free play or backlash in the system. The majority of the the handle travel upwards, pulls the sear, which is a wedge shaped plate through the bottom of the firing pin. As the sear is withdrawn it gradually moves the firing pin away from the cartridge, compressing the spring on the firing pin. When the sear is fully withdrawn the firing pin is no longer restrained by the sear and under the force from the spring, hits the cartridge, starting the ejection sequence. I guess if the handle is partially pulled and then released, the sear will reseat under the forces from the firing pin spring. It might be possible to reach a situation where the sear is resting on the crest of the ramp and is then in a metastable condition and could go either way with the slightest force, one way back to safe, the other way to fire the seat. This is just an engineering guess, I have never tried to reproduce this situation, but I bet a lot of others have over the last couple of years.
As a follow on, the T Mk 2 and later Hawks has a 'PINS' caption on the CWP. If I remember rightly it illuminates if the pins are left in with the canopy closed. It was introduced to try and prevent one of the other hazards associated with seats, leaving the safety pins fitted during flight. The pin operates a microswitch behind the fitting into which the handle seats. If the pin were fitted when the handle is partially pulled, it would not give a PINS warning at the appropriate time. I guess when the crew reach the end of the sortie they probably shut down the electrics before fitting the seat pins so it would not show up at that point. On the next sortie though, after closing the canopy the crew would expect to get a PINS caption but would not if the handle were not in the correct position. It's not how the system is intended to work and its not a very robust means of warning the handle is not safe, even though the pin is fitted. However it is progress in some senses.
Walbut