You have to expand a bit on this. A bug report that consists of a single summary line gives us very little to work with.
It's simple. It is when you paint with a brush in the alpha channel solely without any changing the previous brush strokes on the layer.
Relevant IRC conversation where the feature is defined:
<Pentalis> https://bugs.kde.org/show_bug.cgi?id=300483 is this bug still relevant?, or is there a mechanism to write solely to the alpha channel (other than erasing it with eraser mode)
<bugbot> KDE bug 300483 in krita (Brush engine) "The behind mode for the brushes would be very convenient." [Wishlist,Unconfirmed: ]
<boud> Pentalis: I'm not too sure
<boud> Animtim has asked for something that sounded similar, but we didn't get to a decision
<Pentalis> I think the solution to that request is creating an "antieraser" mode
<Pentalis> Eraser mode lowers the value of the alpha channel, without touching anything else; antieraser increases the value of the alpha channel, without touching anything else
<Animtim> Pentalis: actually that's not what is asked in this request/bug:
<Animtim> he wants to paint only on empty space, it's like painting on a layer behind
<Animtim> (so the "behind mode" name…)
<Pentalis> so even if the pixels have a very faint, 1% opaque color, it shouldn't paint on them?
<Animtim> not really… more like if a pixel is 50% opacity, paint only 50 max opacity on it
<Animtim> if pixel is 100% opacity, don't paint on it,
<Animtim> if pixel is 0% opacity, paint 100% max on it.
<Animtim> really like if you painted on a layer behind, then merge the layers after each stroke
I also assigned this one to myself
Git commit 0963b0a376d73970379ea1782f47b1343cda858b by José Luis Vergara.
Committed on 10/08/2012 at 19:00.
Pushed by jlvergara into branch 'master'.
Add the new Behind composite Op
M +1 -0 libs/pigment/KoCompositeOpRegistry.cpp
M +2 -1 libs/pigment/KoCompositeOpRegistry.h
A +76 -0 libs/pigment/compositeops/KoCompositeOpBehind.h [License: LGPL (v2+)]
M +4 -1 libs/pigment/compositeops/KoCompositeOps.h
It is very handy to draw rocks and leaves.