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.)
Oh that's so clever, to use the water that is there anyway as a means of affecting the dynamics of the pipe. Whoever came up with that was on another level.
Many antennas do this (screwdriver antennas come to mind), so I’d expect some similar ability to do this with exhaust. The only issue I can think of is turbulence from the seams of segments that extend.
I wonder if the exhaust could by dynamic i.e change length depending on the RPMs