Injector

The fuel injector is an electrical solenoid, controlled by the ECU. Fuel under pressure is supplied to the injector, and a pressure differential of about 36 psi is maintained between the fuel line pressure on one side of the injector and the manifold air pressure on the other side.

Inside the injector is a coil. When current is supplied to the coil the injector valve opens, allowing fuel to pass through the unit and into the intake manifold. The ECU controls the current reaching the injector. The longer the ECU sends an "open" signal, the more fuel is injected into the engine.
Although there is one injector per cylinder mounted in the intake manifold just upstream from each intake valve, all the injectors receive the signal from the ECU at the same time, so the actual injection of fuel into the passage is independent of the position of the piston (on intake stroke, exhaust stroke, etc) at the time of injection. When the intake valve opens, the fuel has already been injected into the port and is ready to be drawn into the cylinder.
With the intake valve opening to draw fuel some 30 times per second or more at cruising speed the fact that the fuel is not injected at exactly the moment that the intake valve opens is unimportant. (although in my mind it raises some questions about efficency)