Channel Mixer Tool: Button-Option Explanation Please?

[Koyaanis]

Today I had need to use the Channel Mixer to create a B&W image. I had never played with that [*] shaped selection option (in front of all the Gray or RGB buttons, looks like a sun icon). It seems to allow you to change the luminosity (or Gamma?) of a color channel? If you click on the "back arrow" button to set things to default, the red channel is selected at 100%. If you change the red channel slider it behaves as expected. The same if you select the B or G channels. But if you click on the [*] button and then go back to any of the RBG buttons, it converts the image to B&W. The behavior of this tool is rather confusing if you click on the [*] button and then go back to use any of the others. There's nothing in the help file to explain this behavior, or how and why you'd use it that way.

Can someone explain what that [*] button is for and how, when, and/or why you'd use it?

[Hoogo]

I think L-channel is correct.
Pressing the R-button means that you can mix a new red-channel out of the 3 rgb-channels, without affecting the green or the blue-channel. And with the sun-button you mix a new luminosity out of rgb, without changing the color. Looks good when you turn a blue sky to a dark blue/grey one.

I think it's more or less the same as using the channel mixer to create a gray output, creating a working layer with that and setting the layer to "luminosity".