I'm really going to stick my neck out here, and challenge BEagle's statement.
A normal magneto , if turned by hand , can be felt to break the magnetic"grip"
That instant when it "lets go" is when the lines of flux are broken and the current is induced in the windings,,,hence the points-gap is critical, as this IS the deciding factor in getting the timing spot-on for a fat spark, which will only occur if you have the maximum generated current at the instant the points open and collapse the field....that's what induces the HT voltage in the secondary winding.
At cranking-speed, the rotation can be too slow to generate a sufficient primary current....so we have an IMPULSE START.
This is basically a clock-spring in the couplung between the drive and the magneto"innards"....a Pawl engages the spring-mechanism and continued rotation winds the spring until the pawl hits a trip-mechanism which releases the magneto to be "flicked" round by the spring unwinding. The rapid twirlthrough the break-point gives a full spark. the trip-mechanism is set to operate at a point well retarded from the normal "run" timing. The Pawl has a centrifugal weight, such that, should the engine fire, the Pawl retracts and the spring , uncoiled, acts as a solid drive in it's normal "run" advance timing.
If you crank with a "normal" Mag engaged, It will fire aroung the same time as the impulse starts winding up on the other mag...It could fire a "good" mixture and stop the piston dead then reverse it's direction (a "Kick-back" ) OTOH, the impulse timing is late-enough that by the time the cylinder charge is burning and expanding, the piston and crank have gone over TDC and thus the expanding gases push the piston on the right side of TDC.
Note the impulse -coupling is NOT SPEED DEPENDENT (except to disengage) This means it will ALWAYS (unless broken!) be engaged on a stationary engine....no matter how slowly you turn the prop by hand, the coupling will flick the mag round at it's release -point.
IT THEREFORE STANDS THAT IF THE MAG IS "LIVE" IT WILL SPARK.
A broken ground-wire will give this situation, that's why a running "drop-test" is important, it shows the mag is working AND the cut-out is also working!
Summary...Start on impulse and switch to "both" as soon as the engine runs.
Always be aware that a slight movement of the prop , could trip a spark and start an engine.
Sorry Mr. Beags...If I'm wrong, please tell us!