The first time the magnet passes the hall sensor the circuit retards the spark..The Syncro can be programmed to advance either 20 or 28 degrees, and full advance can occur at either 4000 or 6000 rpm...The timing is set at 28 BTDC static either way...Full retard is either 28 degrees, to 0, or 20 degrees, to 8 BTDC..Most of the engines are set up for the full advance at 4000..Below that the timing is retarded by the circuit..It's almost linear...R C ...