Quick Look’s Slideshow (Full-Screen) Mode in OS X El Capitan
Quick Look in OS X El Capitan really shines in its Slideshow (full-screen) mode. When you’re in Slideshow mode, a completely different set of controls appears onscreen automatically, as shown here.
You can start Slideshow mode with any of these techniques:
Hold down Option and choose File→Slideshow.
If your file is already open in the Quick Look window, click the full-screen button.
The Slideshow controls disappear after a few seconds of inactivity; if you don’t see them when you need them, just move the pointer, and they’ll magically reappear.
To exit Slideshow (full screen) mode, press Esc or click the Exit Full Screen button to return to the Quick Look window or the Close Quick Look button to both exit Slideshow mode and quit Quick Look.
When you’re finished with Quick Look window, click the X button in the top-left corner. If you’re in full-screen mode, click the X button in the control bar, as shown, or press Command+Y, which works in either mode.