Nigel,
This is how it works. Let's talk about the case where the back is vertical and the lens is tilted down. Then as you may know, the Scheimpflug principle says that the film plane, the lens plane and the plane of exact focus all intersect in a line. But there is an additional line called the hinge line that is not so well known, despite Merklinger's attempts to publicize it. In the above case, this line is vertically below the lens and lies in the subject plane. The region in focus is a wedge centered on this plane of exact focus and bounded above and below by two (half) planes which also emanate from the hinge line. The angular opening of this wedge depends on the f-stop.
Saying the wedge is centered on the plane of exact focus is not correct in the sense of angular centering, and it requires some further explanation. At any fixed horizontal distance from the lens, consider a vertical plane. That intersects the two bounding planes in two lines, one above and one below. These are equidistant from the plane of exact focus in the vertical direction. Moreover, at the hyperfocal distance, which depends on the f-stop, that equal distance is (very close to) the distance from the lens to the hinge line. Hence by proportionality, the distances above or below the plane of exact focus can be calculated at any other distance.
In any case, since the region of good focus is a wedge, objects near the lens need to be close to the plane of exact focus to be sharp. That means that if there is any significant vertical extent close to the lens that needs to be in focus, you are unlikely to be able to accomplish it with a tilt.
Saying this in words so someone will understand it is not easy. You would be well advised to draw some diagrams to help you get it right. It is possible to learn to use tilts by rote by just repeating what happened to work in some other situation, but it is much easier if you understand the underlying three dimensional geometry.
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