Inertial supercharging effect is the result of incoming fuel/air charge developing momentum greater than intake stroke would generate alone.
It is achieved by the careful design of the shape of the piston head, the valves and cam profile/valve timing which creates a vacuum that pulls more exhaust gases (and some of the intake gasses) out of the engine.
This is immediately followed by a reflected pressure wave timed to force the extra intake gasses back into the cylinder, thus achieving a greater mass of air/fuel mix in the combustion chamber than possible with conventional methods.
[1] The volumetric efficiency is maximized to increase the amount of air/fuel mixture in the cylinder during each cycle.
[4] The timing of the opening and closing of the valves is essential to ensure the air in the cylinder is maximized to create the most power in each cycle.