This is the fundamental question. "PT" is an acronym for Pressure-Time. This name perfectly describes the operating principle of the pump and the system:
Pressure: The fuel pump generates a pressure of fuel and supplies it to the injectors.
Time: The time that the injector's open orifice is exposed to this fuel pressure determines the amount of fuel delivered to the cylinder.
In simple terms, the pump controls the pressure, and the injector's mechanical operation controls the time. The combination of these two factors dictates the engine's power and speed. It's a simple yet brilliantly effective mechanical system.