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With the breaker plate removed, you can see the centrifugal (mechanical) advance, which is a pair of spring-loaded flyweights that advance spark timing as rpm increases. Flyweights move outward with rpm, advancing the rotor and spark. Spring tension, which is adjusted by bending the advance adjustment arms, controls exactly when advance begins to roll in to make the spark earlier. The "10" indicates half the total number of degrees this rotor will advance, meaning 20 actual degrees.