As I understand it, there is a coarse wheel (6ish slots) and a fine wheel (lots more slots, can't remember how many). The coarse wheel indicates top dead center for each cylinder, and the fine wheel indicates where the motor is between each cylinder's tdc. The coarse wheel controls injector pulsing, and the fine wheel controls ignition advance. The missing slot indicates tdc on #1. All this is combined with the rpm signal off the coil - for both timing purposes. Math logic in the ecu then schedules the injection and spark events per the spark advance tables built into the ecu, influenced by the ect and maf and baro and iat sensors and the tps.