http://en.wikipedia.org/wiki/EXUP

this is beneficial to the performance of the engine at high RPM, but detrimental to performance at low RPM. Closing of the valve then creates back pressure inside the exhaust system, forcing down the piston

Back pressure is a must in four stroke as well although 2 strokes would die without an expansion chamber.

__________________
99 Gurkha with OM616 IDI turbo

2015 Gurkha with OM616 DI turbo

2014 Rexton W with OM612 VGT