What a rinse is
Hot water is poured over the leaf, held for a few seconds at most, and discarded. In gongfu practice the discarded liquor is often used to warm the cups on its way out. It is not a wash in any meaningful sense — a few seconds of contact removes very little of anything — and describing it that way is where most of the confusion about it starts. Whether to do it is one of the few genuinely contested questions in everyday brewing, and the disagreement is largely about which of several distinct purposes is being served.
Reason one: wetting the leaf
This is the strongest argument and it applies to compressed, tightly rolled and heavily twisted leaf. Such material has very little surface accessible when dry, and a brief soak that is poured away begins the process of opening it, so the first infusion you actually drink starts from an already-wetted leaf rather than spending its length hydrating. The effect is easy to observe: rinse a tightly rolled oolong and the first proper infusion is fuller and more even than an unrinsed one, and the second is not markedly different from the first in the way it otherwise would be.
Reason two: dust and fragments
Compressed tea prised apart with a pick, and leaf that has travelled, carries fine fragments that extract fast and contribute grip and cloudiness out of proportion to their mass. A rinse takes a portion of that away. This is a modest effect rather than a dramatic one, and it is worth being clear that it does not make anything safe that was not — it improves clarity and reduces early harshness in a cup, and that is the whole of the claim.
Reason three: warming the vessel
The rinse water heats the leaf, the pot and, poured out through the cups, the cups. For a tea that wants sustained high temperature this is not a side effect but part of the point, and it partly explains why the practice is strongly associated with dark tea and roasted oolong and barely at all with delicate green tea. If you are already preheating the vessel separately, this reason has been satisfied and does not need the rinse.
What it costs
A rinse discards a real infusion. For a fine green tea, a silver needle white tea or a first-flush black, the first seconds carry a burst of aroma and sweetness that does not come back, and pouring it away in the name of a practice borrowed from compressed dark tea is a straightforward loss. There is also a category where the first liquor is precisely what people prize, and rinsing it away has no defence at all. The general shape of the answer: the more open and delicate the leaf, the more the rinse costs and the less it buys.
A practical position
Rinse compressed tea, tightly rolled oolong and aged dark tea, briefly and once. Do not rinse fine green, yellow or white tea, and do not rinse anything you are drinking for its opening aroma. Where you are unsure, brew the first infusion and taste it before deciding to throw the next one away — a rinse can be skipped this session and adopted next, whereas a discarded infusion cannot be recovered. And keep the rinse genuinely brief: a rinse held for thirty seconds is not a rinse, it is a wasted infusion.