Tilted large-format lenses take whatever the focus plane is (even if it's not particularly planar) and lean it over such that the lens board, film plane, and focus plane intersect on a line. If the large-format lens has a very flat field (as the biogon types do), then there will be no real loss of sharpness at the center, as long as the subject in the center is on the focus plane.
If the subject isn't planar, however, one must find a compromise position of the focus plane and then stop down to increase depth of field.
This image, for example, was made on 4x5 film using a 90mm Super Angulon. Every part of the subject that sits in the focus plane is critically sharp, and this photo require both tilt and swing. But I still had to stop down to f/45 to achieve the appearance of sharpness over the whole staircase.
Rick "who dripped sweat on the ground glass for 45 minutes making that photo" Denney