Your photo is displayed with a red line running either vertically or horizontally through it.  The portion to the left or above the red line shows your photo at the state selected by the History State selector found within the History Browser; the portion to the right or below the red line shows your photo after the filter has been applied. 

You can move your photo with the Pan Tool or click and drag the red line to observe the effect on your photo.

Click the Rotate Preview button to alternate between a vertical and a horizontal split.