The command creates a selection in the current layer from the Alpha Channel. Opaque pixels are fully selected, transparent pixels are unselected, and translucent pixels are partially selected. This selection is intersected with the existing selection: only common parts of both selections are kept. The alpha channel itself is not changed.
You can access this command from the image menubar through → → ,
or from the pop-up menu which appears when you right-click on the active layer in the Layers Dialog.