The small, fuel return line from the top of the injector to the fuel filter housing , acts as the final pressure bleed at the injector to allow the mechanical injector to close completely/ quickly.
The main injector pump supplies a fuel pressure pulse, in the 2,000PSI range to force the mechanical injector open, then after the line pressure drops as fuel is released into the combustion chamber the internal springs in the injector close the tip and the remaining pressure in the line returns to the filter , so 12 psi will not push fuel into the cylinder.
The return line is metal , painted black, and must be handled with care , kinking it will mess up the whole system.
The spaghetti of twists and turns in the, equal length, fuel lines from the Injector pump to the injectors, is so each high pressure pulse of fuel arrives at the cylinder at the correct time, in relation to every other cylinder.