Copy and Delete at the same time.

If you're archiving a file to disk (let's say to an external FireWire drive for example), you can drag the icon of the file you want to archive directly to that drive and the Mac will write a copy to that drive. However, your original file still lives on your current hard drive. If you want to have that file deleted from your drive as soon as it's copied to another drive, just hold the Command key as you drag your icon, and the Mac will do two tasks for you — copy the file to the new drive and delete the original from your drive.