I had to make a rollover image: a copy of an image with slightly different colors.
Editing with Gimp 2.6, I just copied the image and altered the colors, but then I adjusted the transparency of the original image (smoother transition from opaque to transparent) and I wanted to copy the transparency information to the rollover image. For some reason, I didn't want to redo the color change again.
After fumbling a bit and searching the Net, as always, I found out a feature I always overlooked: Layer Mask.
That's what I was always wanting: a way to change transparency (the so-called alpha channel) finely, better than using the Eraser (which is one-way, anyway).
So I thought I should share this new knowledge... The procedure is simple:
Display the Layers Palette: Windows > Dockable Dialogs > Layers, or Windows > Recently Closed Docks > Layers, Channels, Paths [etc.]
Right-click on the layer in the Layers palette -> Add Layer Mask
In the displayed dialog, I chose: Transfer layer's alpha channel, then click on Add button.
This supposes your image already has an alpha channel, you can add it with Layer -> Transparency -> Add Alpha Channel.
The layer mask is displayed as an icon on the right of the layer icon. The white border shows it is active: tools will act on it. You can click on the layer itself to change the drawing target and click back on the mask.
Right-click on the layer's icon -> Show Layer Mask. This shows a gray-level image: white is opaque, black is fully transparent, grays are variable amount of transparency. You can use all the Gimp tools to alter this transparency: brush, bucket, etc.
Uncheck the Show Layer Mask to see the effect (you can also paint directly on the combination, of course).
You can temporarily disable transparency: right-click -> Disable Layer Mask (that's a toggle). You can delete the layer mask to make the image / layer fully opaque (no confirmation, but undo works, of course).
You can choose in the context menu Apply Layer Mask to apply the changes made to the alpha channel (and remove the layer mask icon).
You can copy the gray-scale image to the clipboard. Then you can switch to another layer, in the same image, or a different one, display the layer mask as explained above, and paste to transfer the transparency information.
It is a very powerful feature, quietly "hidden" in the context menu of layers. I bet there are lot more I have to discover...
I hope this brief explanation will be useful for you.