Tamping

A tamp creates a level, uniformly dense puck that water has to pass through evenly under 9 bars of pressure. An uneven or under-pressed tamp will channel — letting water shoot through one weak spot — and produce a sour, under-extracted shot. Modern espresso work has de-emphasized tamping force in favor of distribution tools that level the grounds before tamping, but tamping remains the final step before locking the portafilter into the group head.