It is a technique that maximizes the depth of field by rendering
more of the foreground in focus than would ordinarily be achieved
when the lens is set to focus on infinity (when the focus is turned
to the maximum distance). Consider that depth of field is defined
as the distance from a foreground point to a background point that
is rendered as acceptably sharp (it cannot truly be a distance "in
focus" as some would say since it is only possible to focus on one
plane). If you really understand depth of field scales on the lens
barrel, you should be able to comprehend that it should be possible
to have the far point fall just on infinity rather than seemingly
extend beyond it.
Hyperfocal distance is achieved by lining up the centre of the
infinity mark on the focus scale with the indicator line for the
apeture being used for the shot (the one at the far distance end of
the aperture scale, not the near distance end of the scale). The
foreground point of acceptable focus is then estimated by observing
the reading on the distance scale that lines up with the other
aperture indicator line I just referred to. Of course, when you
look through the view finder, neither the near point or infinity
will appear to be acceptably sharp if you are viewing at full
aperture. The effect can only be pre-viewed if your camera can stop
down the lens to the f-stop being used for the shot without taking
the shot. Naturally, this effect is not worth the effort unless you
are going to take the shot F11 or F16 or an equivalent for the film
format you're using.
Micron
A rule of thumb is to think that if you focus on the hyperfocal
distance you will be in focus from half the hyperfocal distance to
infinity. Few zoom lenses have depth of field scales (the Tamron
80-200 is one), so you would have to rely on depth of field
charts.
I used it many times with fixed focal length wide angle lens on
a roll film Mamiya 120 slr. I set the infinity mark in the f22
position on the lens and found I had a depth of field that
stretched from two feet to infinity!