The starting process is well explained here
Flying the SE.5a | The Vintage Aviator
Starting is rather easy once the system is learned and as always every aircraft seems to have its own process. In this case, once primed we simply select both magnetos and also select the booster magneto on and keep clear of the prop. Then the pilot winds the booster magneto and if we are lucky the engine comes to life on its own without the need to swing the propeller! If it doesn’t, we must resort to the usual “Armstrong” starter, where the engineer must swing the prop by hand.
Some cars of the era had a similar method of starting.
Clarification: The whole idea of the starting magneto was to avoid the necessity to hand swing the prop, and the inherent dangers so attached.