Sorry for the delay! I haven't been to the Groups lately, and only noticed your question today.
The technique makes use of perspective, or rather parallax: the fact that perspective is different from two different points of view.
If you put a stake in the ground, another one some distance behind it, and have a look them from some distance, you'll notice that the two move relative to each other when you move your head from side to side.
Put the two at the same distance, i.e. next to each other and do the same, you'll notice that they don't move relative to each other (but still do relative to anything in front or behind them).
So to determine whether two things are in the same plane, all you need is to have a look at them from two positions in another plane.
In cameras, that means you do not have to have a ground glass to 'catch' the image and show whether it is sharp or not. A clear glass screen will do, meaning you'll have a viewfinder image as bright as possible.
But that clear screen needs a mark, a reference line, to indicate the plane of focus.
If a lens is focussed to project an image of something in that plane indicated by the reference line, that line and the image of the thing your are focussing on will remain stationary relative to each other when you move your eye from side to side. Anything that is not in focus however will then move, relative to the line.
So the technique involves moving your head/eye from side to side constantly, watching while you do the thing you want to put focus on through the viewfinder, looking how it moves relative to the reference line. If it doesn't move anymore, you stop turning the focussing ring and know it is in perfect focus.
Though it requires that you concentrate on what you do (and maybe some getting used to), it's easy, and very accurate.
It's a bit easier to do on the large screen of a LF camera than when peeping through the finder of a MF or 35 mm camera. But it works equally well on all formats and with all types of finders. But you have to have a clear glass focussing screen with reference lines.
So all you need to focus is a reference line in the plane of focus. Put that on a clear glass screen, and the image will be as bright as it can possibly be.
Now we can of course see through a clear glass screen, so no mathher whether the image of something is projected in front of or behind the screen, we can see it as sharp.
