What is the Dot3 texture combiner supposed to do with the alpha channel?

I can't figure out what it's doing, but whatever it is, it doesn't make sense to me. Here's a simple example:

Shader "Dot3 Primary" {

Properties {_MainTex ("Texture", 2D) = ""}

SubShader{Pass
{
    SetTexture[_MainTex] {Combine primary Dot3 primary, primary}
    SetTexture[nothing] {Combine previous alpha}
}}

}

Why do I see the alpha channel of the texture instead of pure black? Also, why, if I use an actual Color for primary, does something else happen? (It's some kind of multiplication or something.)

Not 100% sure but I believe it’s a dot product operation. It treats the 2 colors as vectors and finds the dot product between them. Also, I think it treats 0.5 as “zero” or the “center” in texture combiners.

So…

Color value 0 means -1

Color value 0.5 means 0 (the center point)

And a color of 1 means 1