Fuel injector recirculation, how the fuel is circulated when engine is stopped?.why it is done?

Fuel recirculation injectors are fitted with sliding sleeves fitted around a part of the needle push rod held in place by a lightly loaded spring.needle push-rod is hollow and drilled with holes and oil passes through the needle holes when the sleeve is in the lower position to circulating area and the low pressure regions. when
fuel p/p starts delivery the load exerted by the spring is overcome and the sleeve is lifted covering the holes alloving the fuel to pass into the annular space around the fuel valve needle.when the engine is stopped and the fuel pump is not discharging fuel( ie plunger is in lower position and roller is riding the base circle of cam). the fuel circulating pump pressure causes the fuel to pass through the top of the pump ram into the fuel piping and to the injection valve and fuel is recirculated by the sliding sleeve. When the engine is stopped, circumferential movement of the pump ram brings the vertical slots inline with the inlet and spill ports, so fuel can be circulated. fuel is circulated only when circulating pump is running and maintain pressure to overcome frictional losses. circulation helps to keep injector cool and fuel at right temperature to be injected.