Stewart-MacDonald has been doing short videos of various things each month and I didn't get on this list till I ordered a phase switch several months ago. Some of these are very interesting and enlightening for the guitarists out there.
 
This particular one focuses on a non-intrusive tone circuit used to allow customers to evaluate tone before any modification is done. Very slick approach, and the focus is not to have the volume knob suddenly dig into the tone when it move from 10-0.
 
This newsletter is found here, and can subscribe to the list if desired at that link.

Interesting. What it's doing is making your volume control nonlinear, frequency-wise, so that as you turn the volume down a capacitive shunt bypasses the potentiometer for high frequencies. You're essentially combining a tone control with the volume control that makes the signal brighter as you turn the volume down. 
 
The obvious question: why don't guitar manufacturers put in the shunt themselves? 

That is a good question, and I think ultimately that might come down to appeasing traditionalists, both in appearance and performance. Many features require an "on/off" capability to them to achieve this, and folks do not want to buy an expensive guitar which has a different "look" than its predecessors. When I redid my guitar, I added 6 switches, but made painstaking efforts to "hide" 5 of them. Only one is visibly different from the original appearance.
 
Another part of manufacturing environments is that SPC (statistical process control) is the driver, assuming that A=B=C, etc. In this particular case I do not see the harm in that circuit being "stock," but others may. An interesting manufacturing point was brought up by the luthier who PLEK'd my guitar. When I mentioned Gibson PLEKing, he had said "Sure, they do, but with the guitars unstrung (not sure if that is true, or he meant "all strung with the same gauge strings"). Simply stringing them with your choice of strings/tuning can knock this off-kilter quickly, and I have re-PLEK'd very expensive, brand new Gibsons." The manufacturing aspect has to "assume" a standard, but not all fall into this mold.

bitflipper
 
The obvious question: why don't guitar manufacturers put in the shunt themselves? 

Some do from time to time. I have seen Les Paul's of certain model years have it as a feature.
My old Strat had it built in from the factory.
Some people are very traditional and don't like it. Jazz guys like that it gets bassier as you turn down the volume.

I've a Lite Ash Tele that has a resistor/capacitor shunt combination across the volume pot from the (Indonesian or Korean) factory.

It kind of works to let more treble through as the volume pot's turned down, but to be honest I'm often inclined to snip it off and get rid of it. Adding more top as you increase volume can be very useful, unless you're the kind of player who sets the guitar controls to 10 and leaves them alone after that.

And it's a very bright Tele to start with, the last thing it needs is more treble all the time, especially when you turn the volume up to compensate for volume loss when rolling off the tone control.

Do any of the guitars which have this "stock" come with a switch to bypass it? I assume no, since altering the appearance tends to be "sacrilege," but am curious about this.

On a guitar forum there would be 1000+ posts on this already
 
1: This works
2: Some people like it
3: Some people don't
4: People love to disagree about it.
 
Try it on your guitar and see if you like it. If you do like it, solder it in and forget it's there. All he did in this video was put a couple of alligator clips in rather than solder.
 

mettelus
Do any of the guitars which have this "stock" come with a switch to bypass it? I assume no, since altering the appearance tends to be "sacrilege," but am curious about this.

 
It's the most basic of mods on a guitar. Generally - you like it or you don't. It's not usually something you would toggle on/off so it doesn't normally justify an added switch.

I made this mod on one of my guitars many years ago. None of my current guitars have it. I like the clip on concept though.
 
At some point, I may make a clip on version and try it.
 
I have two Carvin tube amps and with the same guitar and cables, they react differently with the volume being turned down.
 
One would benefit from the mod but I don't think the other would.
 
Still a very interesting idea to try before you solder :-)

The configuration would definitely come into play. I tend to use the volume knob very little and attenuate at the amp/interface. There was a point where I debated removing the volume pot completely (having read of some folks who have done this), but realized that has saved me more than once on feedback loops.
 
I do have a couple guitars with tone/volume per HB (the "4-knob jobs") with stock pickups that would most likely benefit from this, so may try the clip-on approach one of these days.

I have grown accustomed to the effect that turning the volume down does to the tone.

Now I notice this affect much more on single coils. I have a G&L that when I back off the volume from 10-8 there is a noticeable tone difference. On my LP I don't notice until I get the 6-7. And it's not as drastic a change as the single coil.