Refer to figure.
The impulse magneto coupling consists of a spring-loaded clutch mounted between the drive spindle from the engine and the magneto shaft.

During engine start, whilst turning over at low speed on the starter motor, the magneto would not have enough rotational speed alone to create a good spark in the spark plug, so the spring absorbs energy through the rotation, then dumps it all at once to accelerate the magneto fast enough for a spark to be formed. This also retards (delays) the spark a little, to make sure that the piston has started the power stroke before the spark hits, as if it ignites the fuel too soon at such low engine speed, it could cause a backfire.

After the engine has started, pawls within the mechanism move out under centrifugal force to prevent the coupling from operating.