How do you add a second hard drive?

Hello, guys! I was browsing eBay and found that there's a company that's buying new iMacs, upgrading the RAM and hard drive, adding a second hard drive and upgrading the SSD. Then they charge about an extra $1500 and selling is new and giving their own warranty. I thought this was pretty interesting, but I'm really confused as to how they would add a second hard drive to this model. I used to be an Apple Genius and worked on these machines, but I can't recall where there would be room for another drive. I'd love to see if anyone has done this or if they have a idea as to how.

Here's the link: http://www.ebay.com/itm/27-APPLE-iMAC-3-...