The tongue detects a small number of basic tastes, and everything else that gets described as flavour arrives through the nose. The route it takes is the interesting part.
Two paths to the same receptors
Aroma reaches the olfactory receptors by two routes: inwards through the nostrils, or upwards from the back of the mouth as a person exhales. The receptors themselves do not distinguish between them.
The second path is called retronasal olfaction, and it operates while wine sits in the mouth and for some seconds after swallowing. It carries the bulk of what drinkers report as flavour.
Both paths end at the same receptor sheet, but the brain interprets them differently. Orthonasal input is filed as smell and retronasal input as flavour.
The tongue's contribution is narrow
Taste receptors detect sweet, sour, salt, bitter and savoury, and in wine that reduces mostly to sweetness, acidity and bitterness.
Everything described as cherry, herb, smoke or citrus is aroma, not taste. None of it can be detected by the tongue at all.
This is why a heavy cold flattens wine to a set of textures and a sour edge. The receptors doing the descriptive work are simply unreachable.
Warming and swirling in the mouth
Holding wine briefly in the mouth raises its temperature and increases the volatility of its aromatic compounds, pushing more of them into the retronasal path.
Drawing air across the wine while it sits on the tongue accelerates the same process, which is the reason professional tasters make an unattractive noise.
Exhaling through the nose after swallowing or spitting is the step that actually delivers the information. Skipping it discards most of what the wine offered.
Why the finish is measured this way
The persistence of a wine is judged after the liquid has gone, when only retronasal aroma remains circulating.
A long finish means aromatic compounds are still being released from the surfaces of the mouth and throat many seconds later.
Short finishes indicate lightly aromatic wine or compounds that bind poorly to those surfaces. Length is therefore a physical measurement rather than a compliment.
What this means for tasting practice
Assessing the nose before tasting and again after gives two different readings of the same wine, and disagreements between them are informative.
Wines that smell more impressive than they taste are common, and the gap usually points to aromatic compounds that do not survive contact with the palate.
Recording both separately, rather than merging them into one impression, is what makes a note useful to somebody who has not smelled the glass.