Question about "Luma is Alpha" mixing mode

[gpvillamil]

I understand that "Luma is Alpha" takes the brightness of a clip, and uses it to set opacity (lighter is more opaque). However, I notice that the slider actually fades between the two clips as well, which is not necessarily what I want.

What I would like is for the black parts of the uppermost clip to be transparent, so that the bottom clip "shines through" at full brightness, and the bright parts of the uppermost clip are fully opaque, and bright.

However, if I turn the opacity all the way up for the upper clip, the bottom clip disappears, even in the black parts. At 50% opacity for the upper clip, the bright parts of the upper clip are dimmed, and the lower clip, where it "shines through" the black parts, is also dimmed. I can't seem to get a situation where white parts of the top and bottom clip are seen at full brightness.

The Luma Key mode works the way I want, but with a hard edge.

I suppose what I'm after is something like a "smooth Luma key" effect, where the borders of the upper clip are smoothed using a Luma Is Alpha type transform. I think Alex May built a FreeFrame mixer plugin that implemented this.