In a normal two-stroke engine, negative pressure develops in the crankcase during the upstroke of the piston. This negative pressure sucks air through the carburettor, where fuel is added, and the fuel-air mixture is then sucked into the crankcase. On the next stroke, as the piston begins it's descent, pressure in the crankcase begins to rise. Because the transfer port opens, most of the fuel-air mixture gets forced into the combustion chamber, but without a modified air intake some of the mixture will try to escape back through the carburettor.
The extra long intake manifold contains a certain mass of air. When the negative pressure develops in the crankcase, this air gets sucked towards and through the carburettor, as before. However, as the crankcase pressure begins to rise again, the momentum of the mass of air within the extended manifold causes it to keep moving towards the engine, to a greater extent than with the unmodified engine. The result is that more of the fuel-air mixture gets to the engine, especially at lower revolutions. This in turn results in more power, manifested as more torque, especially at lower revs.