According to the Etsy seller's description, you grease the inside of the tube and then let the dough rise inside until it entirely fills the tube. The bread is baked right in the tube, so you end up with a perfectly round loaf of bread.
The method for getting the dough into the tube sounds like a magic trick:
Take a 20 in. length of wax paper and fold it in thirds lengthwise. Place the roll of dough on the wax paper (toward one end) and then pull the other end of the paper through the tube. When the roll of dough is where you want it (filling the middle of the tube) roll the tube over so the dough is on the bottom and the wax paper on top; this releases the waxed paper and you can pull it out.
We're tempted to pick one of these up just for the novelty of it!
Check It Out! Pyrex Bake A Round Bread Tube from Pegalee, $15 on Etsy