Rinsing with water straight after brushing feels like completing the job. Chemically it undoes part of it, because fluoride's effect depends on what remains behind rather than on what happened during brushing.
Fluoride acts at the surface over time
Fluoride is not absorbed into the tooth during the two minutes of brushing. It works by being present in the fluid around the tooth while mineral exchange occurs.
When acid dissolves mineral from enamel, fluoride present in saliva promotes redeposition and forms a more acid-resistant mineral than the original.
That process runs over hours, so the relevant quantity is how much fluoride remains available after brushing ends.
A reservoir forms in the mouth after brushing
Some fluoride is retained in plaque, on soft tissues and in a thin layer of calcium fluoride-like deposits on the enamel surface.
These reservoirs release fluoride slowly back into saliva, maintaining a low concentration well after the toothpaste is gone.
Rinsing thoroughly with water removes much of the reservoir before it is established, lowering the concentration available afterward.
Spitting and rinsing are not the same action
Guidance in several countries suggests spitting out excess paste without rinsing, which leaves the residual film in place.
The difference is a matter of how much water is introduced. A small amount of remaining paste is the point rather than an oversight.
Rinsing with a fluoride mouthrinse is different again, since it replaces one fluoride source with another rather than washing it out.
Timing around other products matters too
Using a fluoride mouthrinse immediately after brushing can replace a higher concentration with a lower one, so separating them is often suggested.
Eating and drinking soon after brushing clears the mouth in the same way water does, shortening the exposure period.
Brushing before bed is emphasized partly for this reason, because salivary flow falls overnight and the retained fluoride is not cleared quickly.
Where individual advice is needed
Fluoride concentration in toothpaste varies, and appropriate products differ by age, particularly for young children who may swallow paste.
People at higher risk of decay, including those with dry mouth, appliances or a history of frequent cavities, are sometimes advised differently.
A dentist or physician can advise on what is appropriate for a particular person, and questions about children's fluoride use belong with them rather than with general guidance.