Sometimes you want to try things out before sending them out to the main audio and video outputs. This is what the previewing feature is for. We can use previewing to take a peek at the composition, layer or clip without actually playing it out.
The Preview Monitor window sits in the bottom left of the screen, below the main Output Monitor. Make sure the preview is expanded (by clicking the small triangle at the left) before using it to preview video.
To preview the composition, layer or clip, click its name on the layer strips. You will now see the video in the Preview Monitor. If you have set up preview channels for audio (In the Audio tab of the Preferences window) you will hear the audio of your preview through the channels that you have selected.
If the clip has an alpha channel, you will see the transparent parts on a checkerboard background. This setting can be adjusted in the Video tab of the Preferences Window.
When previewing, the volume and opacity parameters of the composition, layer or clip are ignored.