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I have been thinking about barrel adjusters and I don't see how they can be truly effective although they appear to work, except on this particular bike where they don't seem to do anything.

Is there some exploded diagram or video available their internal workings?

When I observe them they only seem to push against or relax the brake housing to accomplish their function but that doesn't seem enough unless the millimetric movement is enough to adjust the position of the brake pads.

I thought the barrel pushes against something within the brake lever that effects an additional pull or relaxation within the brake lever housing, but an exploded view of a Shimano brake lever I have seen shows nothing of the kind, and that wouldn't work with inline barrel adjusters.

Are there some barrel adjusters which effect some additional pull within the brake lever housing itself, or do they all work via pushing or relaxing the cable housing?

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Barrel adjusters push against the outer cable (cable housing). That's how they work both on gear and brake cables.

The appropriate use of barrel adjusters on all cable brake systems, whether traditional rim calipers and v-brakes, Shimano drum (roller) brakes or mechanical disc brakes, is to pick up slack in the cable that is sometimes left after installation (because it is not always possible/convenient to remove slack while tightening the locknut, e.g., because a "third hand" cable puller is unavailable or the locknut is hard to reach with one), or slack that occurs when a cheaper inner cable stretches out.

The actual brake adjustment (e.g. for pad wear) should not be done with barrel adjusters. It is normally done by undoing the locknut and adjusting cable to correct length at locknut, or by other means that don't involve the barrel adjuster (position hooks, adjustment screws, depending on the specific system/model). Also, when doing any significant cable tension adjustment or other brake adjustment, remember to check the state and alignment of brake calipers and pads (i.e. that pads are not too worn, that they contact the rims at appropriate angle with appropriate surface, and that they do not rub against the tire; same for other systems such as mechanical disc brakes).

Thus, brake adjustment is very different from gearshift adjustment, which is precision adjusted using barrel adjusters.

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    @vfclists Conceptually, brake outer hose is usually slightly compressible (as opposed to gear hose that is more rigid along the length dimension). Typical difference in cable diameters is 0,1 mm (1,5 vs 1,6 mm standard cables). Both these effects are tiny - you should not be noticing them unless you have kilometers of cable length. Roll your barrel all the way in, adjust the cable at locknut, check if there are other ways to adjust brakes (e.g., tension screws or hooks at calipers), pick up the remaining slack with the barrel - that should be all you need.
    – Nik Kharlamov
    Commented Jun 27, 2023 at 12:54
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    In practice, with nice true wheels and not wanting to waste stroke in the levers, you can adjust for pad wear with the barrel adjuster once, then the next time wind it all the way in and use the pinch bolt, then the barrel adjuster again.
    – Chris H
    Commented Jun 27, 2023 at 14:47
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    Barrel adjuster is fine to compensate for brake pad wear, but (with rim brakes) make sure the pads are properly aligned. When they wear down the angle can change and it can lead to partial pad contact or rubbing against the tyre.
    – Michael
    Commented Jun 28, 2023 at 6:58
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    @Robert - as another commenter writes, you can in principle use an adjuster for worn pads (and this would be the appropriate temporary solution e.g. on a long ride/race). But if you only use the barrel adjuster, you might eventually forget to check the brake pads and miss the point where they are misaligned/too worn, or miss problems with other parts of the brake - with risk of catastrophic failure. Bottom line: using barrels for adjustment is doable, but not optimal, and also, as the original poster notes, may not even be enough depending on the specific system.
    – Nik Kharlamov
    Commented Jun 28, 2023 at 8:00
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    Downvoted for the claim that barrel adjusters shouldn't be used for pad wear adjustment. With "modern" brakes that damage the inner cable, you can tighten the brake anchor bolt just once. That's it. If you need to tighten it again, it's unsafe to use the old damaged inner cable, you need a new inner cable.
    – juhist
    Commented Jun 28, 2023 at 17:48
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All barrel adjusters act like a piece of adjustable cable housing and effectively increase or reduce the cable length. As pointed out by Nik, they are only intended for fine adjustments. The big adjustment should be done by loosening the cable clamp bolt, pulling more or less cable through and then tightening the cable clamp again. Cable tension/length should be adjusted in a way that firmly pulling the brake lever doesn’t make you run into the handlebar.

There are some brake levers which have adjustment screws or wedges to put the lever closer to the handlebars to accommodate smaller hands. The disadvantage of this adjustment is that it’s like slightly pulling the brake lever, so it moves the brake pads closer to the rim/disc “in idle” which can cause brake rub (especially if your wheel/disc is out of true or the brake pads not moving symmetrically).

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The reason barrel adjusters with V brakes don't work well is that V brakes are long-pull brakes. This means a given pad motion requires more pull from the cable. Their cable pull is about 2.2 times as much as it is for e.g. cantilever brakes.

Thus, V-brakes require more screw threads from the barrel adjusters. Unfortunately, many barrel adjusters don't have long enough threads. For example if you use drop bar V brake levers that don't have barrel adjusters in the lever and are instead shipped with a noodle having a barrel adjuster, the barrel adjuster is not long enough. In practice you need three: one in the noodle, another after the noodle in a Jagwire Mickey adjuster, third in a Shimano SM-CB90 inline cable adjuster.

I disagree with the accepted answer claiming that actual pad wear adjustment shouldn't be made by barrel adjusters. The reason being that modern brake manufacturers are so afraid that someone could tighten a brake anchor bolt too little that they put features in the brake that crush the cable, ensuring better operation with inadequate bolt tension. However, the very same feature that allows tightening the bolt too little without danger, damages the cable after the bolt has been tightened. This means that unfortunately with these "modern" brakes you can only tighten the cable anchor bolt once. After it has been tightened, the cable has been damaged. If you need to untighten the cable anchor bolt and tighten it again, you need to throw away the inner cable and use a fresh one.

Of course this is totally stupid. Of course brake manufacturers should create brakes that don't crush the cable near the anchor bolt. However, because lawyers are today in charge and are afraid of all possible ways the products can be misused, we can't have such reasonable brakes. Thus, we need to buy brakes that damage cables when tightening anchor bolts, meaning the only reasonable way of adjusting for pad wear is barrel adjusters. In drop bar V brake systems, this means two adjusters may not be enough, you may need three of them.

Cantilever and caliper brakes don't suffer from this, since they are short pull brakes so short barrel adjusters work just fine.

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