You may be familiar with the Match Frame command -- it looks at the clip under your playhead, and loads the corresponding original source clip into your viewer.  If you're not familiar, you can get a quick idea by playing around with in it Final Cut (the keyboard shortcut is simply the letter "F"). 

This function can be useful in an incredible number of situations: for a quick example, if you place only the video track of a clip, then later decide that you need the audio too, you could go the intuitive way or the easy way.  You could dig for the original source, then spend 5 minutes trying to line up the audio tracks with your video ... or you could use Match Frame to bring up the original source clip, already marked with in and out points corresponding to the clip in your timeline. 

By default, Match Frame applies to the top layer of video in your timeline.  But what if you wanted to match to a clip lower in your stack of video clips -- or to an audio track?  The answer is simple, if a little unobvious: after you place your playhead, simply use the arrow tool to select the track you want to target.  The Match Frame command will give priority to your selection, and match to that track's original source.