Hi Ben, I'm sorry for not making the link to my browser extension more obvious in the question. I think the free space page explains it well enough (with images).
You're right - a simple copy-and-paste does it. But the point here is to make life a little easier. We can make the process much more simple. We can get the sources from a family member's profile without opening another browser tab/window. They can be right there on the same page ready to put into the right place without even copying and pasting - just a click will do it.
From the Edit page, the difference is something like this:
Now
1. Scroll up to the family member's name on the edit page.
2. Open a browser window/tab.
3. Got to the Edit tab.
4. Scroll down to the biography and find the citation.
5. Select and copy the citation.
6. Go back to the first tab/window and paste the citation.
(7. If it's an inline citation, click the 'C' button or type ref tags)
The future / with my extension
1. Choose a family member from a drop-down list.
2. Find the citation in the list on the right.
3. Click a button to add the citation either inline (with ref tags) or in the Sources section (preceded by a *).
Just for example - and this is what prompted me to write this code - someone asked me if we could have the citation for a marriage ready to paste into the sources section when we add a spouse. Yes, we can do that. When you create a new profile, the source citations from the first person are ready to add to the profile of the second person (marriage, census, or whatever), with just one click - no copying and pasting, just a click.
About your idea for doing this the other way around - having a button next to the source... Where would that be? On the profile page? It's an interesting idea, but, I imagine, much more complicated in terms of the procedure and the code. I could be wrong, I'm just trying to imagine how that would work. Would they be added to the Sources section of other profiles without even opening a tab? I guess this is kind of what happens with Family Search, isn't it? I could definitely support that one. I'm pretty sure this would be possible, too, in most cases (though I couldn't do it with Javascript).
You mentioned each source needing a separate location, maybe to build a database of sources, and I know I suggested that elsewhere, but it doesn't necessarily have to be that way. For this feature, I'm just getting the inline citations from the biography text and then parsing the Sources section according to *s. If people (incorrectly) haven't used an asterisk in their Sources section, my code won't work well, but when they do, it works fine (it may not be perfect, but it's fine).
I hope that makes it a little clearer.