To understand tilts, you have to understand the geometry. With the standards parallel, there is one plane, also parallel to the standards, which is exactly in focus, but there is a whole region extending in either direction which is still well enough in focus so detail will appear sharp. That region is called the depth of field. It is bound by a near plane and a far plane, which are also parallel to the standards. The extent of the depth of field is dependent on where you focus and the f-stop. The more you stop down, the larger it will be.
When you tilt the lens, this geometry changes radically. Suppose you tilt downward. Then there is a line below the lens, called the hinge line. The exact plane of focus passes through it, as do an upper plane and a lower plane on either side of it, and the depth of field is the region between them. This region is often described as a wedge. As you focus by moving the rear standard, the whole wedge swings on this hinge line, which explains the terminology.
At any given distance from the lens, the vertical extents on both sides of the exact subject plane are equal. But because of the wedge shape, it is quite small close to the lens and gets larger as you move away from the lens. How large it is at any given distance depends on the aperture. The smaller the aperture the larger it is.
You can find graphical descriptions of this geometry at www.trenholm.org/hmmerk/#HR as well as at www.largeformat.info (which this forum is part of.)
But the picture you see at that website and most everywhere else ignores one important fact. You can't see the entire wedge, just that part of it visible on the ground glass in the frame. For example, you will never see the actual hinge line or even anything close to it on the ground glass.
Another point is that the comment that you can do everything just on the basis of what you see on the ground glass, while basically correct, is something of an oversimplification. The reason is that much of what you do is with the lens wide open for accuracy in focusing, but the picture will be taken topped down at a smaller to get the right depth of field. (See below.)
How do you use this in practice and how does it relate to what you see on the ground glass?
First, you should always try to see if you can accomplish what you want by stopping down. In that case there will be no limit vertically on what is in focus at any distance. But there are various reasons you can't stop down without limit. The most important one is that smaller apertures require longer exposure times, which you don't want if subject movement is an issue. In addition, loss of sharpness due to diffraction gets to be more of a problem as you stop down, and of course, there is a smallest aperture---usually f/45 or f/64 for 4 x 5 lenses---which is available.
In that case, you can try to bring things, usually close to camera, in focus by tilting. But that will only work if the region you need in focus is constrained fairly close to an appropriately chosen exact subject plane, particularly close to the lens.
So, how do you go about this in practice? Let me describe how it works in an example. Suppose you have a field of wild flowers starting near you and going into the distance. Suppose you don't have any trees or even bushes with significant vertical extent close to the camera but you do have trees in the middle distance and perhaps mountains in the distance. You want the flowers at ground level in focus as well as the trees and mountains. The bottom plane of the wedge would be at ground level and the hinge line would also be there. You can now imagine a plane starting at the hinge line and passing through the top of the trees or the mountains, whichever is higher. That is the top plane.
Now that you have the wedge firmly in mind, you need first to set an exact subject plane somewhere within it to choose the correct tilt angle. It doesn't matter too much which plane you choose, but in this example the ground with the flowers would seem appropriate. You then tilt to get everything in that plane in focus, doing this all wide open, without stopping down. I won't describe how to do this because it depends on where the tilt axis is and how you go about it. Details can be found at many other places including at www.largeformat.info. But when you are done, you have fixed the tilt angle, which you don't later change, except possibly for fine tuning the result.
But you don't necessarily use the exact subject plane you just used to set the tilt . Instead you focus so, as best you can tell, the exact subject plane splits the vertical extent of the wedge in half. (Remember that as you focus, the exact subject plane and the wedge about it will rotate about the hinge line.) In this example, you would refocus so the exact plane of focus passed halfway up the trees or the mountain, whichever were higher. When you do, the ground level with the flowers would most likely go out of focus.
Your next problem is to determine how much to stop down. There are several ways to do this. One way, which is completely visual is to stop down, observing what comes into focus as you do so. (You may also want to refocus to center the exact subject plane better.) When you have everything in focus, stop, and use that aperture to take your picture.
Unfortunately, this may not work because the image will get dimmer as you stop down. Most people can't see much of anything below f/22. There are several ways around this, but I will just describe one.. Stop down as far as you can and still see well enough. Pick some one point and look at what is in focus on the ground glass in the vertical plane at that distance. This will be a strip running across the ground glass. Measure the height of that strip, and then measure the height of what you would like it to be. Multiply the f-number you are using by the ratio to get the right f-number. For example, suppose everything you want in focus where the trees are lies in a window of height 30 mm. Suppose you have stopped down to f/16, and the window of what is in focus in only 15 mm high. 30/15 = 2, so use 2 X 16 = 32 as your f-number, i.e. stop down an additional two stops to f/32 to take the picture.
There is another method, which I use based on focus spread between high and low planes, but although it is simpler in many ways, I won't try to describe it here.
This process is a bit oversimplified. First, although in principle you first set the tilt angle and then leave it unchanged, in practice, you may want to fine tune it afterwards. Similarly for the position of the exact subject plane.
Finally, let me remark that the better you focus, the better your results. It is impossible to focus exactly, but you can improve your focusing reliability in various ways. The most obvious is by using a loupe, provided it is not so strong that you confuse surface features of the screen with the image. A 4 X loupe is probably strong enough.
A loupe will also allow you to see the dim image better, but there is one problem with that. A loupe lets you see things better, so it decreases the effective DOF as it would be seen in the print. That means you will end up choosing an aperture smaller than you really need. If that also leads to overly long exposure times, it can be a problem.
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