What is fuel injector dribbling & secondary injection? Causes and effects

Reason for leaking are after long periods of service, needle valve and seat will wear(cat fines) along with worn nozzles can cause incorrect atomization an dribbling, springs can weaken with repeated operation leading to the injector opening at a lower pressure than designed./ dribbling is the condition in which fuel
continues to emerge from the nozzle at pressures too low to atomize properly,t is caused by bad seating face or slow closure/ secondary injection happens when the pressure wave caused by the main injection is reflected back to the pump and then again to the injector, reaching sufficient pressure to reopen the injector at relatively late stage in combustion/dribble and secondary injection can cause smoking,lo will be diluted with fuel or in soluble impurities