Most of you know how to link to a YouTube video. You go to the YouTube page and copy the link in the address bar (or, alternately, click Share below the video and copy the link).
You can also simply add #t= and the number of minutes and seconds at the end of the URL. For example, if you wanted to start at 4 minutes and 7 seconds, add #t=4m7s to the end of the URL and you're done!
In the example link below, I added #t=0m13s to the end of the URL, which will start the video 13 seconds into it, or right before the rocket launches.
http://www.youtube.com/embed/m6nTibKZLFA#t=0m13s
Thanks to Darrin Goodman for the tip.