Hello, Dunninator. I used to wonder the same thing!!! So I searched the Internet and there are lots of pictures of the inside of pouches, taken during pouchoscopies. It is hollow because you can see the "owl's eye", which is how the top of the two pouch limbs look as the camera peers up into the pouch. The inside is hollow. I believe the fold is cut when the staple "gun" is inserted into the folded pouch and "fired". The two limbs are stapled shut and cut at the same time. Do a Google search for pouchoscopy photos. You can also find on YouTube a 20 minute long video of an actual surgery and formation of the J pouch and stapling. It is graphic, fair warning. I posted the video a few months ago to this site.