Capt will fly the Take off, climb and cruise. Approaching top of descent he will hand overcontrol to the F/o who then flies the descent and approach to decision height. Then the Capt takes control for the landing. On sector 2 the roles are reversed.
It was invented by BA to stop rushed and unstable approaches. You will find mixed views on its benefits in todays modern cockpit but it does work well in low Vis approaches.