Preview provides a quick way for you to preview your effects before rendering them. By default playback is restricted to the affected frames only. You can change most effects and parameters while preview is playing. You can use the scrub bar or transport controls to step through the preview or click play in the transport controls to play continuously. Click stop to stop playback, of course. Other transport controls or clicking the scrub bar also stops playback. The transport controls to seek to the beginning or end of the movie goes to the beginning or end of the selected frames.