The question is Why? The most plausible answer is that commercial soda contains different acids: citric acid, carbonic, and phosphoric acids that can break down stains. The longer you leave the soda in the toilet, the more time the acids have to dissolve any stains.
I think this is probably the best use for commercial sodas and pops 🙂
To see Katie’s tutorial, click the Using Cola as a Cleaner link below.