What I know, "alpha" slider works with both "Filter Active" and "Mask Selection" checkboxes checked.
The "Filter Active" checkbox enables the options: red, green, blue, hue, saturation, value, fade, alpha.
If "Mask Selection" is not checked "alpha" will not work.

Take a look at the video "Cin_BluBanana-alpha", please:
https://files.fm/u/d3t4uf3kyc

IgorBeg