Bosch EFI injectores are on/off solenoid valves. When you turn the key to position 2 power is provided continuously to all injectors on the + side. When the engine starts, the injectors are triggered on/off by grounding the - side in pulses. EFI injectors work at constant pressury via pulse width modulation. The ECU simply times longer or shorter pulses depending on the amount of fuel required. If the polarity is reversed, the injectors will not work. If there was something wrong with the harness the engine woudnt start.
This information is for megasquirt but it applies exactly the same way to any other EFI system.
http://www.megamanual.com/v22manual/inj_ani.gif