Sorry fopr the late reply, I didn't get back down to this thread the last coupla days.
You've got the basic concept down. It's not "entirely" correct to say that the mag or distributor retards the spark for start up. They're typically set up by what's called static timing, which is nothing more than setting them where they need to be (TDC) with the motor stopped. A slight retard at static doesn't necessarily hrt much, but a slight advance is where there can be trouble.
The mag, whether it is an impulse-coupling type like the H4/J4 or the earlier types with the manual advance, are set up so that from a dead stop or at the paltry speed that hand-cranking delivers, they are tripping and firing at TDC. On the H4/J4s, once it's running, the impulse coupling takes over and delivers a constant advance that varies only with the tractor. On a given tractor there will be either TDC or a constant advance.
On a battery ignition, the concept is pretty much the same at low speeds -- i.e., firing at TDC. Once running, the weights take over in the cenrifugal advance set-up and deliver an increasing advance with an increase in engine rpms, with the maximum advance usually achieved at somewhere between 1/2 and 2/3 of the rated max rpms for the motor.
The danger in hand cranking with any advance is if the spark touches off the gas in the cylinder before the piston gets to the top. The aim in starting is to get the piston to go back down on the combustion stroke, with the crankshaft going in the same direction as it was turning, whether by hand or by electrical starter. If it should fire before the piston gets to the top of the compression stroke, it can actually drive the piston back down the path that it just came up on the compression stroke, reverse the crankshaft and the handcrank. That's the kickback that can get John Q. Armstrong into trouble.
A starter motor is considerably stronger, enough to overcome that early ignition and keep everything turning in the proper direction, even if it makes for hard starting. Even if it weren't any stronger, it has the advantage of not having bones or reproductive parts that can be damaged my a reversing handcrank.
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