Acid: the correction nobody teaches you
A cook tastes something, decides it is flat, and adds salt. Tastes again, still flat, adds more salt. Now it is salty and flat.
Flat and under-salted are different problems, and the second one is usually acid.
Acid gives contrast. It cuts fat, pulls sweetness back from cloying, and makes everything read brighter. A long braise or a cream sauce loses definition as it cooks, because sustained heat rounds every edge off. A little acid at the end puts them back.
There is a test that takes ten seconds. Put a spoonful of the dish in a bowl, add one drop of lemon or vinegar to that spoonful, and taste it against the pot. If the spoonful is better, the pot needs acid.
Which acid you reach for matters more than people expect. Vinegar is sharp and clean, and its aroma is volatile, so it fades with heat — add it near the end. Citrus is rounder and fresher, and the zest carries aroma the juice does not; juice added early cooks off entirely. Tamarind and pomegranate molasses bring sourness with body and some sweetness, so they change texture as well as flavour. Yoghurt and buttermilk are mild and will split if you boil them, so temper them in off the heat. Wine is acid plus everything else in the wine, and it needs cooking through — raw wine in a finished sauce tastes raw.
Timing follows from that. Late and off the heat when the aroma is the point. Early and cooked in when you want the acid working on the dish rather than sitting on top of it, as in a tomato sauce, or a braise where it has to balance fat the whole way through.
Go slower than you would with salt. Acid is easier to overshoot and much harder to hide, and there is no equivalent of adding more stock to pull it back once the balance has gone. A little, stirred through properly, tasted, then decide. Two rounds of not quite enough beats one round of too much.
