[gasbag]

The act says that anytime a non-US ship carries goods that are from a US port, including Puerto Rico, it cannot take those goods to any other US port. So instead of being able to do a sequence of [Barbados, Puerto Rico, Florida, Barbados], a ship would have to instead follow [Barbados, Puerto Rico, Barbados, Florida, Barbados]. It doesn't eliminate trade entirely, but it definitely increases costs--especially in PR itself, which has to rely only on US ships for US goods, creating a monopoly.