Some camera backs are designed so that tilts and swings pivot at a line on the focusing screen. Furthermore, that line is offset from the center of the focusing screen. The procedure for determining tilt (and swing) appears to be easy: Focus so that the part of the image on the line on the focussing screen is sharp, then tilt (or swing) so that the other part of the image (usually indicated by another line on the screen placed opposite to the first) is sharp. The image should still be sharp on the original line since the screen should pivot around that point.
The specific cameras that have this feature are the Ebony's with U-backs and several Sinar models.
My question is is this a useful feature or a gimmick