Howard Bond offers reasonably priced workshops on the technique as
applied to conventional printing.

An old post on the subject:

From: "Nicholas O. Lindan" <nolin...@ix.netcom.com>
Subject: Unsharp masking: What & How
Date: 1997/12/30
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Newsgroups: rec.photo.darkroom

Unsharp masking accentuates the detail in the picture.  It does
not make the picture sharper.  It is the photographic equivalent
of turning up the treble control on the stereo.

The results can be quite striking, though, given the right
subject matter.  Rocky landscapes become surreally bold while the
sky and clouds look unchanged.

To explain the process I will use a picture of a light gray
square on a dark grey background.  Thrilling subject, no?  And I
am going to take a line through the middle of the picture on
which to make graphs of either reflectivity or negative density.

In the beginning we take a plain old B&W picture:

Subject reflectance:

 Lite grey           _______________________
                    |                       |
                    |                       |
                    |                       |
 Dark grey _________|                       |_______________

Negative density:

 Lite grey _________                         _______________
                    |                       |
                    |                       |
                    |                       |
 Dark grey          |_______________________|

So far, so good.  If we make a print from the negative we would,
if all things are done right, get the same tones as the subject.

Instead we make an unsharp mask by contact printing the negative
onto another piece of film, called the mask, and doing it badly.  We
place a material between the negative and the mask when we expose
the mask.  This makes the mask into a fuzzy positive image of our
subject.  Since the mask is fuzzy, the technique has come to be
called 'unsharp masking'.  We also expose and develop the mask so
that it's contrast is less than the negative.

If we plot the density of the mask it looks like:

 Very lite grey      ______________________
                    /                      \
 Lite grey ________/                        \_____________

Notice that the transitions between the grey tones is sloping.
In real life the slope will look more like an S-curve.

If we now sandwich the negative with the mask the densities of
the two will add as shown below:

Negative density:

 Lite grey _________                         _______________
                    |                       |
                    |                       |
                    |                       |
 Dark grey          |_______________________|

Mask density:

 Very lite grey      ______________________
                    /                      \
 Lite grey ________/                        \_____________

Resulting density:

 Grey
 Dark grey ________/|                       |\______________
                    |                       |
 Very dark grey     | _____________________ |
 Almost black       |/                     \|

The masked negative we have created has a lower contrast for the
continuous regions than the original negative.  However the
contrast at the transitions between the two greys have the same
contrast range as the original negative.

To compensate for the loss of contrast we print the masked
negative on a higher contrast grade of paper with the following
result.

Print reflectance:

 White
                    |\                     /|
 Lite grey          | \___________________/ |
                    |                       |
                    |                       |
                    |                       |
 Dark grey _______  |                       |  _____________
                  \ |                       | /
 Black             \|                       |/

Which you can compare with the original subject reflectance:

 Lite grey           _______________________
                    |                       |
                    |                       |
                    |                       |
 Dark grey _________|                       |_______________

In the print the transition from dark grey to lite grey goes
first to black then takes a flying leap to white before settling
down to the lite grey tone.

This effect is sometimes called 'sharpen' in photo/paint computer
programs, but actually no sharpening takes place.  What does
happen is that edges are made more prominent.

Note that I cheated in the drawing of the print reflectance in
the final step.  The horizontal dimensions of the overshoots
should be half of what is shown, but ASCII only has one slope
available for the /\ characters.

Sure thing "Lynn".