Way back when Jet Skis had 2-stroke engines, you could buy an aftermarket exhaust pipe with water injection in a couple key spots. This allowed the pipe to be dynamically tuned to different RPMs by changing the speed of sound within the pipe.

The stock pipe was tuned to a specific RPM, allowing the reflected exhaust pulse to return to the cylinder just as the port was closing and “slam the door” on the fresh air/fuel charge.

With variable water injection, you could have more than one optimal RPM.

(All this is from memory as I read the ads in Splash magazine.)