Activating graphics and video on scroll-down

I’ve seen several sites where the graphics, especially video, activate once they are scrolled into view. Facebook has been doing this for a while with videos. How is this done? Have you come across any resources that teach this? I’m sure there is a term for this, but I don’t know what it is.

Apple does it now (10/28/2014) here:

Most of the tutorials I’ve seen are for HTML5 video. Unfortunately, my vids are in iFrames.

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