How does this visa-free transit working?

You can use 24 or 144 hours visa free transit while in Shanghai. Your citizenship is eligible, but you have to fulfill also the transit criteria. Your flight has to be from one country, let's say Canada and your outbound flight has to be directed to some other country, let's say Cambodia. If these criteria are fulfilled, that you can apply for 144 hours visa free transit at the counter at Shanghai airport upon arrival. It is necessary that you present your airline tickets and accommodation bookings in printed form and you don't pay anything. Also, keep in mind that your both flights cannot have any other landings within mainland China beside Shanghai. Hong Kong, Taiwan and Macau are fine for transit because they are considered as third countries for transit purposes.