I was taught (by an ex-RAF instructor who had flown in WWII) that it was for 'sucking in' and to call out 'sucking in' before swinging the prop (with the fuel turned on but the mags off). Once this was done, the swinger called out 'switches on', the person in the cockpit threw the left mag and replied 'contact'. The prop was then swung again to start the engine. I was flying Austers and Tiger Moths at the time and they didn't have starter motors, they were all hand swung.
Why was just the left mag used when starting?