Lets say you have a clip in your timeline that has linked audio and video and you want to move just the video to a higher (or lower) track. When you click on the clip both the audio and video are highlighted so both will want to move. What can you do?
The long roundabout way is to highlight the clip and tap Command + L to unlink them, then move the bit you want up or down (Option + Up or Down Arrow is good for that) then highlight both the audio and video again and tap Command + L again to re-link them. That's too much. There is a simpler way.
01) Hold down the Option key.
02) Click once on the audio or video you want to move.
03) Let off of the Option key. Notice that just the audio or video is now highlighted, yet they're still linked? (meaning the names are underlined which indicates it's a linked clip)
Great, now drag that bit up or down as is your want and then click elsewhere and you're done.