Check this menu selection to toggle display of the alpha channel for the image currently displayed in a view window. This option will be disabled if the image does not contain an alpha channel or if the image is in 8-bit mode or less.
Note: Holding down the Shift key while selecting a channel to display will cause ThumbsPlus to display only that channel. All others will be toggled off. The default behavior is to toggle the selected channel on or off depending on the current state.