When you skim effects, in the viewer you can see the effect applied to the shot. Nothing new here. But if you press and hold Alt key, it freezes the current frame and you can continue skimming other effects (i.e. looks) and see them applied to this particular frame.
Apple's idea is slightly different:
From the Final Cut Pro X Help - To preview changing the effect’s primary control: Hold down the Option key while moving the pointer over a video effect thumbnail.