What stays behind
A vessel poured until it stops running still holds liquid: soaked into the leaf itself, pooled under the leaf bed, and held in the spout and around the filter. In a small heavily loaded gongfu vessel that residue can be a meaningful fraction of the infusion. It is also the most extracted portion of it, since it has been in contact with leaf throughout and has not been diluted by anything. Leaving it there is not a small rounding error in the schedule — it is a concentrated extract sitting on hot wet leaf, continuing to work.
Why it keeps extracting
TeaHQ's own reading of well-attested general knowledge, with no single source behind it.
Wet leaf between infusions is hot, fully hydrated and in contact with liquid. Every condition for extraction is still present; only the volume of water has changed, and reducing the water while keeping the leaf makes what remains stronger rather than stopping the process. By the time the next infusion is poured in, that residue joins it already deep into the late, grippy part of the extraction sequence. The result is a second cup that is harsher than the first, arriving with a timing that appeared correct — which is why the fault is so often attributed to the tea being delicate or the schedule being wrong.
It compounds
The error is not constant across a session, it accumulates. Each infusion adds its own residue, and each residue is more concentrated and more late-fraction than the one before, so a session that started well degrades progressively. This produces a very recognisable pattern: two good infusions, a fair third, and then a cliff. Anyone whose sessions consistently fall apart at the fourth steep should suspect the pour before the leaf. A tea capable of eight infusions rarely announces its exhaustion by turning sharp; it announces it by turning thin.
How to actually empty a vessel
Pour until the stream stops, then tip further and hold — the last of it comes out slowly and is the part that matters. With a gaiwan, angle the lid to open the gap wider at the end. With a teapot, lift the lid or open the vent so air can enter and the liquid can leave. Rest the vessel tilted, mouth downward over the tray or waste bowl, between infusions rather than setting it upright. Some practitioners give the pot a firm final shake, which is worth doing and worth doing over something that catches drips.
Vessels that cannot be emptied
Some equipment makes this impossible by design, and that is a reason to choose against it rather than a technique problem. A plunger press holds the leaf under the liquid permanently. A basket infuser left sitting in a mug keeps the leaf wet in the drink. A pot with a deep sump below the spout retains a pool that no amount of tipping clears. Where you are stuck with one of these, decant the tea into a separate vessel the moment the steep is done — moving the liquid away from the leaf achieves the same end from the other direction.
Between sessions
If leaf is to be returned to later in the day, drain it thoroughly and leave the vessel open so the leaf can breathe rather than sitting sealed and damp. A few hours at room temperature is ordinary practice; leaf left overnight in a warm kitchen loses its aroma and is not worth brewing for flavour whatever else is true of it. The same reasoning applies to the vessel itself, which should be left open to dry — a closed damp pot, particularly an unglazed one, will eventually smell of it.