Inertial supercharging effect is the result of incoming fuel/air charge developing momentum greater than intake stroke would generate alone. It is achieved by a combination of head/port configuration, and cam profile/valve timing.
Inertial supercharging effect in two-stroke engines
The inertial supercharging effect from two-stroke engines is the result of the design of an expansion chamber in the two-stroke exhaust system. Its carefully designed shape and length uses the expanding exhaust gasses to create 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 than normal induction methods. Expansion chambers only work well at a narrow engine speed range which is why two stroke engines are referred to as having a "powerband". Since the early eighties exhaust powervalves have been developed which have the effect of altering the timing and/or volume of the expansion chamber, greatly improving the spread of power of high output two stroke engines.