YouTube is a fun site and offers lots of handy features. When we start to build things in Second Life ™ we run into some challenges. YouTube gives us some help.
See: YouTube Embedded Players and Player Parameters. As you scroll down the page you’ll eventually find the commands to add to a URL to get the video player to do what you want.
There are people that have figured out lots of tricks that can be done with the YouTube player. For instance, see: 10 YouTube URL Tricks You Should Know About.