Why stirring does anything at all
As material dissolves off a leaf surface, the water immediately around that surface becomes locally concentrated, and the closer the local concentration gets to the surrounding water’s, the slower the transfer. In still water that concentrated layer sits where it formed and has to disperse by diffusion, which is slow. Moving the water sweeps it away and replaces it with fresher water, restoring the difference that drives extraction. Agitation therefore raises the extraction rate without touching temperature or time. Anyone who has ever dunked a teabag has run the experiment: the same bag in the same water for the same duration gives a visibly darker cup when moved.
Where the water lands
Pouring directly onto the leaf from a height stirs the bed vigorously and wets everything at once. Pouring gently down the inside wall of the vessel lets the water rise under the leaf with far less disturbance. Both are used deliberately: the hard pour for compressed or tightly rolled leaf that needs breaking open and wetting through, the gentle pour for fine broken leaf or delicate buds that would otherwise be knocked about and over-extracted. Notice that the choice is being made whether or not you are aware of it, since a kettle held high and a kettle held low do different things to the same tea.
Agitation and astringency
The reason gentleness is a virtue with some teas is not aesthetic. Vigorous movement breaks fragile leaf and creates fines, and fines have far more surface area per gram than intact leaf, so they extract fast and disproportionately contribute grip and cloudiness. That is the mechanism behind the standard advice not to squeeze a teabag or to press the leaf with a spoon: you are not extracting more tea so much as generating and then wringing out the harshest available fraction. Stirring a delicate green tea has the same character on a smaller scale.
Pouring from height, and what it is for
Several traditions pour from a considerable height into the serving vessel, and the reasons are physical rather than theatrical. It aerates the liquid, it cools it towards drinking temperature on the way down, it mixes the pour so the last of a pot is not stronger than the first, and in Moroccan practice it raises the foam by which a well-poured glass is judged. Mongolian salted milk tea is ladled and returned repeatedly for the same combination of mixing and aeration, and Hong Kong milk tea is poured back and forth through a cloth for that plus the filtration. In every case the pour is part of the recipe.
The consistency argument
TeaHQ's own reading of well-attested general knowledge, with no single source behind it.
The practical reason to think about agitation is repeatability. If you swirl the pot vigorously one day and leave it undisturbed the next, you have changed a variable without recording it, and the resulting difference will get attributed to the leaf or the water or the weather. Pick one habit — swirl once after pouring, or never — and keep it while you are adjusting anything else. This is the same discipline that makes single-variable adjustment work, applied to the variable people forget they have.
Where the effect is small
It is worth keeping the size of this in proportion. Agitation is a genuine lever and a modest one next to temperature and ratio. In a long western steep the leaf circulates on its own as convection currents form, so deliberate stirring adds relatively little. In a twenty-second gongfu infusion, where the pour itself is a large fraction of the total contact time, how the water goes in matters much more. As a rule the shorter the steep, the more the pour is part of it.