The natural frequency of a Helmholtz resonator depends upon the volume of the flask and the diameter and length of the tube attached to the flask.

A tuned two stroke pipe is a sophisticated Helmholtz Resonator.  The exhaust port and the first section of the pipe (head pipe) where the angles are small, can be compared to the tube attached to the flask of a Helmholtz Resonator.

Suzuki's power valve system on the LTs adds volume to the head pipe and the exhaust port.  Adding length and or volume to a Helmholtz resonator lowers the natural frequency.  Lowering the natural frequency of the tuned pipe is the same thing as tuning the pipe to be in tune at a lower RPM. 

Suzuki's power valve system essentially fools the engine into thinking that there are two different pipes.  The engine runs on the pipe that is tuned for low end up to about 5500 to 6000 RPM and then is quickly switched to the pipe that is tuned for good power above 6000 RPM