That is what the diagram seems to show. On the base of the large cylindrical part of the pump, inside the crankcase, should be a fair-sized piston which is exposed to crankcase pressure fluctuations. Crankcase induction two strokes, like just about all chainsaws, have quite heavy crankcase compression once every revolution of the engine, changing to slight suction in the opposite half of the revolution. Those pulsations act powerfully on the piston in the base of the pump, causing it to reciprocate.
