What is the logic behind fuel-injection timing?

In normal carbureted or injected engines it is only the vapor mixed with air that actually combusts. Most people do not realize that in a common port-injected gasoline engine, the fuel is aimed right at the back of the intake valves where it completely vaporizes (almost instantly) when the fuel hits the back of the hot intake valve. So the start-of-injection timing is set so all of the fuel hits the back of a closed intake valve. This happens at light to moderate loads for best efficiency. This strategy changes at higher loads (or for cold-cranking) when the injection timing gets changed to open valve injection for best power and knock resistance.