As Zathos said, pull the baffle and the entire breathing is easier giving a leaner not so efficient burn producing less power hence the drop in revs.
If you had a very dirty air filter then you might get the revs rising when you pull the baffle because it would drop from overly rich to just right, this time giving more power and the revs rising.
I did some of this at collage back in 76, there is nothing new. It is all to do with the speed of sound, and its not guess work, look at the old race cars from yester year with their great long thin Triumphs and the short wide Triumphs on more modern ones.
You can only tune for one rev, so you have to choose where you want it.
Either at peak HP or peak Torque would be the best place to tune for and you would want to set the inlet length to match so that you get a positive pulse hitting the inlet valve just as it opens and a negative one hitting the exhaust valve, again just as it opens. Sounds easy, eh.
The pulse travels at the speed of sound, (which varies with atmospheric conditions) you get one reflected pulse (+ive to -ive and visa versa) from each diameter change and one from the end of the pipe. When the exhaust valve opens the pressure wave is positive and travels down the pipe to the first diameter change where it is reflected and attenuated, part of it carries on to the next diameter change and so and so forth until it hits the end of the pipe where the final positive pulse is reflected. Every time one of these waves hits a diameter change it is reflected 180 degrees and attenuated, also a 90 degree bend will attenuate the pulse by 50%. There are hundreds if not thousands of variables to effect and negate the effect of all that perfect pipe tuning. The same thin is also going on on the inlet track only it starts out as a negative pulse when the inlet valve opens.
Other things to look at are 4 into 1 for peek power and 4 to 2 to 1 for mid range (I think but it could be the other way round.
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As said above this only works for one rev. Say on a 1000cc bike peak power is at 13300 rpm. You would tune for that. At 13299rpm or 13301rpm you would get no added help. The bad news, at certain revs the timing of the pulses are such that a positive pulse hits the exhaust valve just as it opens and negative hits the inlet valve just as it opens, look at the lumpy power and torque curves for bikes from only a few years ago. Newer bikes have midrange boosting power valves in there exhaust, (in effect changing the length of the exhaust,) variable inlet valve timing as well as variable inlet tract lengths all in aid of putting this effect to good use allowing a range of revs to be covered rather than just one.
The SET valve in K6/K7 Suzuki's exhausts is to keep the noise emissions down
as Dyno test of before and after the valve is disconnected show no difference in power or torque in the mid range.
