In a two-stroke motorcycle engine, how does the crankcase act as a pump during operation?

1. The downward stroke of the piston pressurizes the fuel-air charge in the crankcase, forcing it up through transfer ports
2. It uses a separate mechanical impeller driven by the clutch basket
3. It uses a vacuum-operated diaphragm mounted inside the oil sump
4. The rotation of the flywheel magnetically forces the fuel mixture into the cylinder