“By relaxing the Jones Act, you provide more opportunities and more vessels for goods to get an affected area,” explains Marc Fialkoff, an adjunct professor at Virginia Tech’s School of Public and International Affairs who has studied the law. “People get goods faster, because there aren’t that many U.S. vessels.”

Yet it’s tough to gauge the impact of the 10-day waiver announced Thursday. In the immediate term, Puerto Ricans are struggling to get food, water, medicine, and other basic necessities, but that’s not necessarily because they aren’t on the island. Once those goods reach Puerto Rican ports, it’s proven challenging to distribute them around an island whose roads, electrical grid, communications lines, and other basic infrastructure have been destroyed or badly damaged. Nor is clear how much excess capacity Puerto Rican ports can take, though Fialkoff cautions that should not slow any waiver.

The government issued a short-term Jones Act waiver after Harvey and Irma, but initially hesitated after Maria. It said the earlier waiver was necessary because without tankers of fuel reaching the Gulf Coast, gas prices would have skyrocketed. “We have a lot of shippers and a lot of people who work with the shippers who don’t want the Jones Act lifted,” President Trump said Wednesday.

But the president seems to have bowed to the political reality that issuing a waiver was politically popular. The problem facing mainland politicians is that there is a great deal of public pressure to show effort to help Puerto Rico, even as many of the steps under consideration don’t have clear, immediate impacts. The result has been a steady flow of arguments that Trump waive the Jones Act, often with little explanation of what immediate impact the step would have.

American carriers who supply Puerto Rico under the Jones Act contend that’s because there wouldn’t be an impact. “The biggest challenge is how you can move the cargo,” Jose Ayala, vice president of Puerto Rico services at Crowley, one of those carriers, told The Wall Street Journal earlier this week. “The cargo is here. The people of Puerto Rico should not have any fear that there is not going to be food or medicine on the island.”

That’s accurate as far as it goes, say some experts. “I think that's true this week, but that's not going to stay true,” says Salim Furth, a research fellow at the conservative Heritage Foundation who has argued that the federal government should waive the Jones Act for Puerto Rico as long as federal aid is flowing to the island. “We need a long-term waiver. If they’re using mainland taxpayer money, they should be able to use American goods.”

Here’s the problem, as Furth sees it: Rebuilding Puerto Rico will require huge amounts of goods like steel, concrete, cinder block, and lumber. But transporting those goods on the existing Jones Act fleet will cause various problems. For one, the fleet already runs a tight schedule moving everyday goods to the island, and it’s not clear that it can add huge shipments of building supplies without crowding out basic supplies, nor that the barges that make the runs are well-suited for other cargo. For another, the cost of transporting those goods from the mainland on American-flagged carriers will push up the costs, and lead Puerto Ricans to turn to cheaper goods from Caribbean and Latin American countries. As of a 2013 Government Accountability Office report, roughly two-thirds of vessels bring cargo to Puerto Rico are foreign.