Direct Injection uses very fast high pressure fuel injectors. Typical design is a solenoid operated needle and a set of very fine orifices that atomize fuel directly into the combustion area rather than into a pre-combustion chamber where it is mixed with intake air. Newer on the scene are piezoelectric injectors and I have no idea how they meter the fuel but I do know the price goes up.
EDIT
Iwas curious and it did not take long to find the answer. Piezoelectric injectors replace the solenoid with a stack of piezoelectric ceramic material that expands and contracts to actuate the needle.
Unlike traditional port (indirect) diesel injection the ECU introduces an atomized fuel charge directly into the combustuon chamber in multiple very precisely timed pulses per ignition event. This allows for much greater control over the formation of the ignition event. The result is increased torque, better fuel economy, smoother and quiter operation, and more complete combustion with fewer exhaust particulates and gases. The performace gains are very significant - up to 50% more torque and 15% better fuel economy vis-a-vis the sane engine using port injection. DI is also uniquely well suited for operation with turbochargers allowingveven greater gains. VW pioneered that technolgy with their TDI (Turbo Direct Injection) first introduced in 1989.
In order for the ECU to accurately control the fuel delivery it needs to know the individual flow and reaction characteristics of each injector and where its positioned in the engine. That calIbration data is provided by the OEM that manufactured the injector and is programmed into the ECU using software provided by the OEM that designed and built the engine. For over the road vehicles there are many sources for the hardware and software diagnostics. Not so much for offroad applicatuons where the engine OEMs have restricted availability. Last I checked a Diagmaster license, hardware, and software from Kubota will set you back about $10K plus annual renewals. And unless things have changed individual owners cannot buy it.
Dan