Each injector in these engines is its own injection pump. Before there were electronically controlled hydraulic activated injectors, this type of system was know as "Unit Injectors" and were activated by the cam shaft via rocker arms. This is of course opposed to the external injection pump you referred to that was connected to each individual injector via high pressure steel tubing. In both the older systems, injector timing was fixed. Some styles of injection pumps had one "jerk" pump and a distributor type head. Another style had individual jerk pumps actuated by a wobble plate. Others (all copied from the original Bosch system) had a separate jerk pump for each cylinder. Those pumps looked like(still do) a little engine. In our PSD engines injector timing varies according to many parameters that the computer monitors from boost pressure to exhaust temp., RPM etc. It is the advances in electronic control of the fuel delivery system that has allowed both the HP increases and clean exhaust (compared to the older systems) that we have today.
Hope this is a good enough explanation.