Well, the secret is that the print is wearing the glasses!

Actually a lenticular sheet is a series of lenses embossed or extruded as long half cylindrical lenses.  (see illustration below). The images are sliced up and a piece from each is put under each lens, when you view the print the lens lets each eye will see a different view and creates the 3D effect. It will all be explained in much more detail in the tutorial including how you can proof you own with just an ink jet printer and a laminator.

ZaxysDMI

Oh, here's a couple of links.

http://www.peterjsucy.com/Products.htm - some sample animated gifs demonstrating as best as possible via the web what is actually happening.

http://www.extremevision.com - one of the top printers in the country.