Percolation

Percolation is brewing where water flows continuously through a bed of grounds and out the other side, carrying dissolved coffee with it. Pour-over, batch drip machines, the moka pot, and espresso are all percolation methods.

It is the counterpart to immersion, where grounds sit steeped in still water. The defining feature of percolation is flow: the water is always fresh as it meets the grounds, so extraction can be efficient and the cup often comes out cleaner and more articulate. The catch is that flow can go wrong. If water finds an easy route it channels through, leaving most of the bed under-extracted while the channel over-extracts.

Your main levers are grind size and pour technique, both of which set the flow rate. Finer grind and gentler pouring slow the water and raise extraction; coarser grind and forceful pouring speed it up and lower extraction. The drawdown at the end is your readout of whether the flow behaved. Even bed preparation matters more here than in immersion, because flow is unforgiving of unevenness.