Most people are familiar with the phenomenon of PARALLAX. If you look at the same scene from different directions, the relationship of objects in the scene will change if the objects are not in the same place.
When you focus a lens, you move the aerial image projected by that lens until it appears "sharp" on a ground glass/plastic. If the aerial image is in front of, or behind the plane of the ground glass, the image will be out-of-focus.
Reticle focussing uses parallax to determine if the aerial image is in the same plane as some feature on the ground glass, such as a reticle millimeter scale, AF rectangular mark, a metering circle, a grid, or even some dirt on the screen.
Shift your head so that you look in from the left edge, then move again to look in from from the right edge of the viewfinder. The image of the detail you want to be in focus should not shift relative to the reticle mark / AF mark / whatever on the focussing screen. Adjust the lens focus to minimize the parallax shift.
This method works best if the camera is on a tripod, and at high magnifications (especially with viewfinder Magnifier S). It can be used equally for central and off-centre subjects. More importantly, it can be used where conventional focussing aids are unhelpful.
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