There is eccentricity between the driving and driven gear, the oil will be sucked when oil pump is running under driving crankshaft to make the internal and external gear disengaged.
The gear is keeping turning and internal and external gears will be separated by the crescent block.
Thus, the oil in gear flow to outlet and pump out in the raising oil pressure will be carried to friction surface of other engine parts.
Oil pump won't run until engine stop working, so it makes the route for lubrication oil circle round.
And the pressure limiting valve is open or close is controlled by the high or low oil pressure to steady the comprehensive performance.