How does it work
Basically this circuit modulates the MAF output voltage dependent on the duty cycle of the EGR valve. With a working EGR valve, the MAF output voltage is modulated too, but only because the airflow through the MAF is really lowered if the EGR duty-cycle is increased. This circuit doesn't change the actual airflow, but it changes the airflow value that is reported to the ECU.
First the MAF output voltage is buffered by transistors tn_1 and tp_1. The emitter voltage of tp_1 is equal to the MAF output voltage. This is needed as the MAF doesn't seem to allow much load on it's output. The new MAF output voltage is connected to the ECU via r_6. This voltage can be lowered by drawing some current through r_6. This current simulates the opening of the EGR valve. The EGR duty-cycle is converted to a voltage by r_14, r_15 and c_2. The voltage across c_2 equals Battery_voltage*EGR_duty/2. (The divide by 2 is needed for circuit reasons.) This EGR duty_cycle value is converted to a current by tp_4 and r_11. The current is 'mirrored' by tn_2 and tn_3 (r_12 and r_13 are added for increased accuracy) such that the collector current of tn_3 is equal to the collector current of tp_4, only the direction is inverted. Now the signal going to the ECU is modulated by the tn_3 collector current, which is dependent on the EGR duty-cycle.