If you actually bother preemptively reading your software documentation, you may have already known this, but this is a huge time-saver over having to render test movies for my HDTV.
At work, I'm using a 27" 1080p monitor/television as my secondary monitor. We use a model that has a similar color profile for presentations, so using this screen as a preview screen is essential. Otherwise, we'd have to keep our presentation displays unpacked and hooked up to a computer, transfer files, and so on. I won't get into (more) details, but suffice to say it would be a royal pain.
Even with this setup, I've been rendering drafts to an mp4 file and running a video player fullscreen on the HDTV (which matches the production environment). This is still a step I'd rather not have to do, so finally today I did some searching.
'Lo, and behold! There is a way.
According to Adobe documentation,+ ( + on Mac) does the following:
Resize application window or floating window to fit screen. (Press again to resize window so that contents fill the screen.)
Since my composition window is on the second monitor already, I pressed+ twice.
Bonus Off-Topic Chatter
The slashes in a URL are just that, slashes. They are technically forward-slashes, but we just call them slashes. There are no backslashes in URLs.
See also: xkcd