Here's the scenario, imagine you are using the Offset tool to create a copy of a wall offset by four feet. The Copy option that appears on the Options bar is checked by default. That's great, but what if you wanted to use the Offset tool to move the wall not create a copy. You have to un-check Copy to get the Move instead. If you press the CTRL key prior to selecting the wall to offset the check mark is cleared in the Copy Option.
The AUGI post discusses this technique in conjunction with the Mirror Tool. Like with Offset the default setting for Mirror has Copy checked, assuming you want to create a mirror/copy, not a mirror/move.
As you probably already know, you can create a copy of an element by selecting it, pressing and holding CTRL and then dragging the selected element to a new location. When you press CTRL with Offset, Mirror, Copy and Move the Copy option becomes un-checked or checked in the case of Move.
I've never found any mention of this in the help file in the past. I just scanned through the 2010 documentation for any mention of it (using CTRL as a criteria) to see if they've added it but didn't find anything now either. A far as I can tell it's undocumented. Thanks to Aaron for the reminder!
A video might help?