Once the pump runs, your options are narrow. Espresso is a puck-prep problem.
What you cannot change mid-shot
Grind. Fixed at grinding. Dose. Fixed at dosing. Distribution. Fixed before the tamp. Tamp. Fixed.
If any of those is wrong, the shot is already wrong.
What you can change, on some machines
Pressure, with flow control or a lever. Lowering pressure mid-shot reduces channelling and can extend a good extraction.
Stopping point. The one universal control — you decide when to cut it.
Pre-infusion is before, not during
Wetting the puck at low pressure before full pressure lets it expand and settle, which reduces channelling. Set on the machine, not improvised.
Why puck prep matters so much
Water follows the path of least resistance through a compressed bed. Any inconsistency in that bed becomes a channel, and a channel means part of the coffee is blasted while the rest is barely wetted.
That is why the same dose and grind can give wildly different shots.
The prep that actually works
WDT with fine needles — the biggest single improvement. Level the bed before tamping. Tamp level, not hard. Tilt matters, force does not. A distribution tool if you like, though WDT does more.
The diagnosis tool
A bottomless portafilter. You can see channelling happen, which is the only way to know it is your problem.
Last reviewed 8 August 2026