Impulse coupling
A spring-loaded coupling between a magneto shaft and the drive gear inside the engine. When the engine is rotated for starting, the impulse coupling locks the magnet so it cannot turn. The spring in the coupling winds up as the crankshaft continues to turn, and when the piston is near top center, the coupling releases and spins the magnet, producing a hot and retarded spark.
Aviation Maintenance Technician Handbook — Powerplant, p. G-15
Taught in PP-H Procedures, PP-H Quick reference, PP-H Concepts.