The movies are named to include their geometry. (There’s no practical way to get the dimensions from a QuickTime movie without this hack.) So the first filename on this page is first_steps_sm-iPhone_320x240.m4v. The geometry part is coded as 320x240. As long as the dimensions are matched by [any number up to 4 digits][the letter x][any number up to 4 digits] the geometry can be read by the function in this page.
The links to the movies have the classname “popup”, and the script looks for those. Further, the links are directly to the bare movie files. This is why it works on the iPhone and other MobileSafari devices without any extra code.
Links with the popup class are rewritten to have the target attribute, pointed to a hidden iframe. Whenever they are clicked, they show the iframe and quickly size it to the movie and center it on the screen. When you click outside of the iframe, it hides again.
The movies are in an iPod-friendly format, which means they will also play in a QuickTime plug-in in a browser without further ado, too. iDevices can play a subset of the formats and encodings that the QuickTime plugin can play.