By not spending all the money that congress has appropriated. Technically appropriations are a maximum the government can spend, not a requirement to spend tha

By not spending all the money that congress has appropriated. Technically appropriations are a maximum the government can spend, not a requirement to spend that money. However, Congress does not like the idea of the present just refusing to provide all the appropriated funds to some programs, so they wrote the Impoundment Control Act of 1974, which requires the president to make all the appropriated funds available to the specified programs. Note the word available. Programs are not technically obligated to use all the money, but technically that is up to the program's head. In practice programs will almost always use as close to all of the money as they can get, since if they don't use all the funds, Congress might choose to reduce their appropriation next year. Because the constitution seems to mandate mandates paying the debt, and leaves the amount of debt up to congress (Article I Section 8), upon hitting the debt ceiling, the president must continue to pay the debt, and must not issue more debt. But the Constitution does not require making all appropriations available so the Constitution will overrule the Impoundment Control Act of 1974, in such a scenario. Clearly if it is impossible for a president to adhere to both the constitution, and a statutory law, it is the statutory law that gives way, not the constitution itself. So X-day should trigger a partial government shutdown, while the government continues to pay its debt, and fulfill contracts and other statutory payments, but cuts off enough other spending to stop exceeding revenue. This would be similar to what happens when congress fails to pass an appropriations bill.