People have a hard time understanding the electrical sequence for starting the engine and activating the ecu's. This is for a 928S4, GT, and GTS. here it goes:

1. Turn on the ignition switch. That puts 12v on the bust 15 which carries switched 12v to various areas of the car.
2. The bus 15 feeds power to the alarm unit and provided the alarm feels ok (turned in standby state) the power switches on the EZK relay. The EZK power side is connected directly to the battery. It switches the power from the battery to the LH ecu pin #35.
3. When the LH feels happy (self test passed an ready to go) the LH turns on the LH relay over pin #21. The LH relay main  contact feeds power to the  Injectors, MAF, Idle actuator, Tank vent, en Intake Resonance valve etc. The power side of the LH relay is also directly linked to the battery.
4. When all that is done and the engine turns over during starting, the EZK sends a signal to the LH that the engine is turning over, the LH turns on the Fuel pump relay via pin #20 and injecting fuel starts.
5. If all is well, the engine starts.