Where the heat goes
Pour hot water into a cold vessel and the two come towards a shared temperature almost immediately. How far the water falls depends on how much material the vessel is made of and what that material is: a thin porcelain gaiwan takes a modest amount of heat out of the water, and a heavy stoneware pot or a cast iron one takes a great deal. The drop happens in the first seconds, before you have started timing anything, which is why it is so easy to miss. Someone brewing a roasted oolong at what they believe is boiling in a cold heavy pot may be steeping it appreciably below that for the whole infusion.
The teas where it decides the outcome
It matters most for the teas that want the top of the range: heavily roasted oolong, compressed dark tea, robust black tea, and anything being boiled. These want sustained heat to open at all, and a vessel that quietly removes several degrees at the start and keeps conducting heat away afterwards produces exactly the flat, closed cup that gets blamed on the leaf. It matters equally in short-steep brewing, where the whole infusion is over in seconds — a temperature drop that a three-minute western steep has time to recover from partially, a fifteen-second gongfu steep does not.
The teas where a cold vessel helps
For delicate green tea the same effect works in your favour, and several traditions exploit it deliberately. A broad shallow unheated vessel is a cooling device: pouring boiled water into a wide cold bowl and then on to the leaf is the entire mechanism of the Korean cooling bowl, and the shape of a shiboridashi does much the same thing. If you are trying to get water down to gyokuro temperatures without a thermometer, the cold vessel is not an error to be corrected but a tool. What is unhelpful is doing it accidentally and not knowing which way the temperature went.
How to do it
Fill the vessel with hot water from the kettle, swirl it so the walls and the lid both warm, and tip it out. That is the whole procedure and it takes a few seconds. In gongfu practice the discarded water goes over the pot as well as into it, and lands in the tray or waste bowl. Warm the cups at the same time from the same water, because a small cup of tea poured into a cold cup loses heat immediately — the smaller the serving, the more the cup’s temperature matters, which is why cup-warming is standard in exactly the styles that use small cups.
The aroma step you get for free
There is a second reason the practice persists. Put the dry leaf into a warmed, emptied vessel, close the lid, wait a moment and then smell it: the residual heat volatilises aromatics from the dry leaf without any water involved, and the result is often more informative about a tea than the first infusion. This is a routine part of gongfu sessions and it costs nothing. It also has a diagnostic use — a musty or sour note in a warmed dry leaf is much easier to detect this way than in the liquor, where it can hide behind everything else.
When to skip it
For a mug of teabag tea it is a refinement nobody needs. For anything brewed at deliberately low temperature it is often counterproductive. And for glassware there is a real hazard the other way round: thin glass that goes straight from cold to boiling water can crack, so warming a glass gently is a matter of not breaking it rather than of flavour. Everywhere else it is a habit worth having, less because any single occasion is transformed than because it removes an uncontrolled variable from every brew you make afterwards.