This extension does not add any UI element and uses no resource at all. Only on YouTube pages, a very tiny script is injected which controls the player's playing status based on document visibility. When a YouTube tab is brought to front, the player automatically resumes exactly like pressing the play button and when document is hidden, the player will automatically pauses exactly like when pause button is pressed.

For FAQs and other useful YouTube enhancer extensions please visit:
http://add0n.com/youtube-tools.html?from=auto-pause