Ultralights, the pressure pulse is reflected back towards the valve from the end of the pipe and may assist in reducing or increasing gas flow as the exhaust valve closes. I dont quite know the dynamics, but what I do know is that changing exhaust systems meant complete rejetting of carburettors and a changed torque peak position. From memory a shorter stack moved the torqu peak up the RPm scale and a longer pipe did the reverse.
Be aware that there is no such thing as a free lunch and you are not going to suddenly find oodles more power by replacing a competently designed system with some snake oil salesmans product.
Having said that, Rotax has just announced changes to the 912 iS Intake system, along with ECU changes, that radicaly modify its torque curve for the better