There is a long explanation here on starting such a plane here:
http://www.pilotweb.aero/techniques-...wing_1_4043093
I think the key part occurs in the discussion of the the coil spring in the magneto.
"While you are setting the propeller, a spring inside the starter magneto is coiling. (Assuming it’s an impulse mag; some have different arrangements such as ‘shower of sparks’, or none at all.) As you pull the propeller forwards, you should hear a click. This is the spring mechanism letting go, sending a jolt of electricity to the spark plug. If the timing is set up correctly, the plug will fire just as the cylinder passes the top of its stroke and is beginning its way down the cylinder.
Note that it’s the un-coiling spring that turns the magneto and produces the spark, and it does so much faster than your hand on the propeller, which by itself is relatively slow. So if you have the ‘click’ impulse mechanism (most engines do), you don’t need a lusty pull. "
I think the point here is that if the prop was left in a position where the spring was coiled and ready to let go, the slight movement of the prop could release it and "fire" the magneto and thus the spark plug.