I am a serial unlinker. Being able to move audio and video separately is one of the great benefits of professional video editors like Final Cut Pro. I was constantly mousing over to the button bar to click the link icon. That motion is a time waster.
I had recently transitioned to using the keyboard shortcut of shift-L to turn linking on and off. But a another way I found is, with linking turned on, to just hold my option key down and click, lets say the end of a audio clip, then right click the highlight to add a cross fade. Or I can option click between two video clips to highlight then right click to add a cross dissolve. I’ve read a lot of Final cut books, done Lynda.com to death, but somehow missed this little time saver.