That's a good explanation of what it's supposed to do. That was how I understood it as well.

But anytime I've tried it, I've ended up with conflicts where there shouldn't be (like, I already solved that conflict when I merged earlier) and/or completely undesirable results in the end (for instance, some of my changes are just NOT in the result).

So I just gave up on the whole feature. Simpler to just merge the source branch into mine.