That image reasserts how complex and sensitive to change the window region of the whistle is. My first impression is that what's desired is just dialing in an ideal level of air efficiency or reasonable options for blending tonal purity with windiness.

If the windway volume changes by adjusting the floor or the ceiling, that would, you'd think, also might mean slightly adjusting the size of the windway exit and slightly adjusting the aim of the air stream headed to the blade. That might adjust many things, including windiness, sense of backpressure, perhaps volume or efficiency of the blade to do its work, and therefore tone and chiff. Some notes might sound better/worse or easier/more difficult, in higher or lower octaves.

Sounds like a lot of R&D would be needed to find workable options.

I never heard it used for any instrument,it was a descriptive sounding term I somehow ended up using. Maybe many others have also come to use this term on their own. Interestingly the term in common usage as relates to gas/water/etc pipes is ALSO incorrect-see Wiki's definition below.

==== "Back pressure (or backpressure) is a resistance or force opposing the desired flow of fluid through pipes, leading to friction loss and pressure drop. The term back pressure is a misnomer, as pressure is a scalar quantity, so it has a magnitude but no direction. The fluid is what is directed, tending to flow away from high-pressure regions and toward low-pressure regions. If the low-pressure space is more high-pressure than intended (e.g. due to obstructions or tight bends in an exhaust pipe) or the high-pressure space is more low-pressure than intended, this opposes the desired flow and reduces the discharge. Similarly, bending or other operations on a pipe (such as a stock car exhaust system with a particularly high number of twists and bends[1]) can reduce flow rate. "====

So it appears everybody is incorrect using this term including the folks working on pipes-the non-musical pipes.Consequently the objections some have to using this term in relation to resistance in whistles....well,if we understand what someone means by it what's the problem? It's just another word/term from one field adapted in meaning to mean something else in a particular field.

Yeah,it may not be the most accurate use of the language,but how many here use the idiotic term "same difference" to mean "same thing"? Or refer to a cotter key as a cotter pin?

There are words/terms used in Chinese martial systems which while related to the actual meaning of the words/terms do not truly match what the words means in the language. The meanings have been somewhat altered for technical description/usage in the systems. Nobody in those circles worries about it,even though an individual not familiar with these slightly altered meanings could find them somewhat confusing until explained.

"Backpressure" seems to be a fairly common term with whistlers,so even though "resistance" would make more sense and be accurate....again,if I know what you mean on the job when you say "cotter pin"..... What's it matter if we're adopting a term from somewhere else and twisting it a bit to mean something within a different field. I should say "resistance" but I'll continue to use "backpressure".Unless the entire world gangs up on me about it.

Nah,I'll use it anyway!

BackPressure And Resistance
(June 17 2020)

I discovered very early with my first bike
How bicycle inner tubes argue with spite
When pumping air INTO the inner tube
The inner tube pumps air OUT as a rule
It's a mechanical battle like paint-ball balloons
You just see a hint as air pressure fights it's volume
Appearing so complacent as a tire starts to swell
But rebellious in proportion to the pressure dial
Defeating your purpose at any novice mistake
Between flat and exploding tires, you learn to moderate

Just when you think you're done, you hear it
Pull away the bike pump and the creature spits
There's raging BACKPRESSURE there the nozzle nixes
That can rhyme with "flat tire" and tire fix kits
Or explosions if glass cuts you a new nozzle sometime
And you walk to buy a new tube pretending not to mind
If the nozzle, pump, glass or anything slips
And the backpressure ruptures, bub, that's it
It's flip flop flip flop flip flop flip

Then you go to the gas station stoked
Expecting you'll pump up your tires and float
But it just can't function
When the backpressure's ruptured
So you patch, replace or won't be going

My fixed tire was spinning on mile after mile
Until I was out there at the mountainside
I played a DX006 of medium RESISTANCE
It's air efficiency a joy with long long phrases
I played WONDER to the cosmos into the night
Just me, my whistle, and the sheet music of starlight
With slow lyrical riffs I played the mountains to sleep
And woke them in the morning with a warm fresh breeze

Some day I'll be older, weaker, thinner
But I know I aim to keep on with long note riffing
And I know what my strategy to cope will be
I'll have a whistle with resistance suiting me
So if I can keep my bike at the ready
My whistles on a trail of musical discoveries
The inner tubes and lungs conditioned and healthy
Backpressure and resistance in a harmony

(composed by me, today, June 17 2020, looks like it needs
to be accompanied by whistles, walking bass, ambient nature sounds,
maybe concertina and a roaming cello playing two-note chords with
a strong sense of rhythms indicating motion, anxiety and bliss)