On the Vectra system (the only one I have had dealings with)the emulator was just a box with a couple of relays and some load resistors in. When the vehicle switches to gas, the gas ECU literally turns off the petrol injectors by switching over the relays but to avoid injector codes being set in the petrol ECU the same relays switch in a set of dummy load resistors. i.e.the petrol ECU still thinks it iscontrolling the fuelling but in reality it is doing nothing.