A federal shutdown is not a decision to close the government. It is the automatic consequence of a legal rule that bars agencies from spending money Congress has not appropriated.
Spending requires an appropriation
Under the constitutional arrangement, money may be drawn from the Treasury only through appropriations made by law, and a long-standing statute reinforces this by prohibiting agencies from incurring obligations in advance of one.
When an appropriation lapses, agencies therefore cannot lawfully continue activities that would create a financial obligation, including paying staff to perform them.
The prohibition is what produces the shutdown. Nothing separate has to be enacted; the closure follows from the absence of an authorisation rather than the presence of a decision.
Not all funding lapses at once
Appropriations are enacted in separate bills covering different parts of the government, and it is possible for some to pass while others do not.
A partial shutdown affects only the agencies whose bills have lapsed, which is why some departments continue normally while others suspend activity.
Programmes funded on a continuing basis outside the annual cycle are unaffected, so major benefit payments generally continue even while the agencies administering them operate with reduced staff.
Excepted activities continue unpaid
The statute allows work to continue where it addresses the safety of human life or the protection of property, and each agency maintains a plan identifying which functions qualify.
Employees performing those functions continue working without pay during the lapse, while others are furloughed and prohibited from working at all, including voluntarily.
The distinction is legal rather than a judgement of importance, and it produces the familiar pattern in which security screening continues while inspections and permitting stop.
The costs are largely deferred, not avoided
Back pay for the lapse period has been provided in past instances, so the wage saving is generally reversed once funding resumes.
Meanwhile the shutdown itself consumes resources, since orderly suspension and restart both require work, and contracts interrupted mid-stream can carry renegotiation costs.
Delayed permits, applications and inspections create backlogs that take longer to clear than the lapse lasted, which is where much of the economic effect sits.
Continuing resolutions are the usual avoidance
Congress frequently extends existing funding levels for a short period rather than completing the full appropriations bills, which prevents a lapse without settling the underlying disagreement.
Repeated short extensions carry their own cost, because agencies cannot commit to multi-year work or plan hiring against funding that expires in weeks.
The mechanics described here reflect the general framework; specific rules and practices have been modified over time and are interpreted afresh in each instance.