The image is displayed with a red line running either vertically or horizontally through it. The portion to the left or above the red line shows the image 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 the image after the filter has been applied.
You can move the image with the Pan Tool or click and drag the red line to observe the effect on the image.
Click the Rotate Preview button to alternate between a vertical and a horizontal split.
The History State selector can be moved by clicking and dragging to any desired history state.