Without clicking the amazing link provided above a simple explanation for what a power valve does is it raises and lowers the exhaust port heights of the cylinder as the engine RPM increases and decreases to change the port timing and create a more smooth, powerful running two stroke. Snowmobiles have been running exhaust valves since 2001 and maybe a few years sooner. Gives you the best bottom end power and adjusts with the RPMs to give you the best top end power. Most work off cylinder pressure to raise the guillotine valves