A gentle stir evens out extraction.
Dry clumps barely extract. A few gentle stirs make sure all the coffee is in contact with water; stirring hard adds bitterness.
What it is
Anything that stirs the coffee bed: a swirl of the brewer, a stir with a spoon, a fast pour.
Why it matters
Agitation breaks up dry clumps and helps water reach every particle, which raises and evens out extraction. Too much pushes fine particles into the paper and can clog it, making the brew slow and bitter.
In practice
Use it on purpose: a swirl after the bloom and after the last pour levels the bed. If a brew runs slow and bitter, try less agitation before a coarser grind.