Why you see the timing mark on pulley when a Chevy 350 with mallory HEI distributor while using light?

The timing marks on the pulley of a Chevrolet 350 engine with a Mallory HEI distributor can be seen while using a timing light because the Mallory HEI distributor uses a magnetic pickup to trigger the spark. When the timing light is directed at the timing marks, the light will flash when the magnetic pickup passes the timing tab, indicating the point at which the spark plug fires. This allows you to adjust the timing of the engine by rotating the distributor until the timing marks align with the desired timing specification.