What is pre-infusion in espresso?

Pre-infusion is a phase at the start of extraction where water saturates the coffee puck at low pressure before full 9-bar pressure is applied. This allows the grounds to bloom and swell evenly, reducing the risk of channeling. Many prosumer machines have built-in pre-infusion (e.g., Breville's pre-infusion setting, La Marzocco's natural pre-infusion). It generally improves extraction evenness and is especially beneficial for lighter roasts.