I fly atr42z, I may not be able to give a complete picture but would give a basic idea of pw127e. There are two independent ignition systems per each engine (1- ignition excitor - that produces high voltage electrical current, 2- spark ignitor - to produce spark with high voltage current) so, When you select the start rotary selector to any of the setting, the relevant system is ready to spark - except if u start in air, both A and B would be energized irrespective of selection, now as soon as u select the starter Pb "on" the starter/generator rotates the high pressure spool with either battery only, extrnal power or if other engine is running then from 10% to 45% (45 is self sustaining rpm) the live engines electrical power supports the revving up along with the battery. We input fuel via CL between 10-20% nh depending on ITT n as soon as fuel is purged in ( 14 fuel nozzles of which 4 are connected to secondary fuel supply only) the spark ignitors start their action to light up the engine ( in case we don't input fuel or delay, the Max Nh values by starter only would be about 30% approximately). At 45% the starter generator stops Assisting the Nh, which comes back online as a DC generator at 61.5% Nh. Until the CL is not in AUTO the PL will directly govern fuel and hence the Nh.