It takes an almost constant time for the flame front to propagate so if the engine is turning slowly the spark needs to occur later than when it's turning fast. One mag has an impulse coupling which delays the ignition timing at cranking RPM. Many (most?) piston-engined aircraft use the same system. If there are separate switches the check list will tell you which to use for start; if you have a key switch the selection is done inside the switch in the start position.
Many years ago I hand swung an aircraft not knowing that the chap in the cockpit had selected both, the engine fired backwards and I received two broken fingers (A rude word was uttered) :-(
HFD
Last edited by hugh flung_dung; 18th Jan 2018 at 09:32.
Reason: typo and incomplete