Relegation from the Spanish top flight is a financial event before it is a sporting one. The revenue that disappears is largely broadcasting money, and the spending ceiling falls with it.
Broadcast income drops by a wide margin
La Liga's television rights are sold collectively and distributed across the division, and the second tier's arrangement is worth a fraction of the same share.
The reduction lands in a single season rather than gradually, so a squad assembled under top-flight income has to be funded under second-tier income.
Commercial and matchday revenue fall too, though usually by less, since supporters and sponsors do not disappear at the same rate.
The spending ceiling follows revenue down
Because the league calculates each club's squad cost limit from its own projected income, a relegated club's ceiling contracts automatically.
Registration rules then bind at the new level, so the club may be unable to register players it is already contracted to pay.
Reducing committed expenditure therefore becomes urgent rather than prudent, and it has to happen inside a single transfer window.
Contracts anticipate the drop
Spanish playing contracts commonly include clauses reducing wages by a defined proportion in the event of relegation, agreed when the player signs.
Those clauses do most of the adjustment automatically, which is why relegated Spanish squads do not empty as completely as the revenue change alone would suggest.
Release provisions often accompany them, allowing a player to leave for a reduced fee, so the better players still tend to depart.
Promotion is not a symmetrical recovery
Returning to the top flight restores the broadcasting share, but the club re-enters with a squad built for a lower level and a ceiling based on recent lower revenue.
Spending has to be authorised against projected income that has not yet arrived, which limits how aggressively a promoted club can rebuild.
This is a substantial part of why immediate re-relegation is common. The financial recovery lags the sporting one by at least a season.
Why the second tier is unusually competitive
Relegated clubs retain larger infrastructure and support than their new rivals, but their squad budgets converge quickly towards the divisional norm.
The result is a league where the nominal favourites are only modestly better resourced than the field, and promotion is decided over a long season and a playoff.
A club that fails to go straight back up faces another year of reduced income, and the gap becomes progressively harder to close.