I do a lot of traditional drawing, so one of the common tasks I perform in GIMP 
is cleaning up the background after scanning a character I've sketched on paper 
(erasing stray pencil flecks and so on).  I've frequently used the Erase (with 
alpha channel) to perform this task, because it means I end up with a 
transparency that I can paint some manner of digital backsplash underneath.

My general process for doing this was:
1 - Duplicate the layer
2 - Perform a full Color to Alpha transformation on the lower layer (white as 
3 - Use the Eraser on the upper layer, with a soft-edged brush, to clean up the 
background.  Any mistakes during the process I can clean up using the Alt 
(anti-erase) modifier.

This yields a smoother foreground-background fade around the edges than simply 
the Eraser on a single layer.  But recently I discovered the faster way of 
doing this task:  The "Color erase" painting mode, which performs a 
Color-to-Alpha transition as part of any other drawing operation.  So I don't 
need to worry about managing multiple layers anymore.  However I do need a 
function to perform the inverse, so I can compensate for mistakes...

...and it dawns on me that the "Behind" paint mode is exactly what I'm looking 
for here.  All I need to do now is set up a keyboard shortcut to toggle between 
the two modes and I'll be good to go.

Anyway, it should be noted in GIMP's online manual 
(http://docs.gimp.org/2.6/en/gimp-tools-paint.html#gimp-paint-mode-examples ) 
that the "Behind" and "Color Erase" blending modes are inverse functions of 
each other, with "color erase" transforming solid color to alpha and "behind" 
transforming alpha to solid color.  (Should I file this on the bugtracker?)

-- Stratadrake
Numbers may not lie, but neither do they tell the whole truth.
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