You can use the Quick View Window in Final Cut Pro in order to preview effects or composite images in real time without having to render them constantly.  Depending on the computing power of your Mac, the clips in your sequence may become sluggish as you change motion parameters and add effects.  Quick View will play slowly through the clips and cache the frames to your computer's RAM within the specified range.  After caching the frames, it plays the clips in real time.  Doing this avoids having to create a render file, which can add up and take up serious space on your hard drive if you don't manage them.  We came across someone recently who had 1.5 TB of render files...ouch!

To use the Quick View Window, go to the Tools Menu > Quick View or you can use the keyboard shortcut Option 8.  The Quick View opens up a window called the Tool Bench; it is a free floating window that you can move around to wherever you want.  Notice the View pull-down menu gives you a choice as to which Final Cut Pro window you want to output to Quick View.  Adjust the range slider and click the play button.

You can also use this tool when you're zoomed in to an image to see how the final composition plays without changing the zoom level of the Canvas; and to preview filter changes on the fly, you can use the Quick View tab.  As you preview the effect, you can enable and disable individual filters in the Viewer Filters Tab to see which filters suit your clip and which do not.  You can even make changes to filter parameters as Quick View plays the clip.  This is really helpful in previewing filters that may require rendering or that don't play every frame in real time.