Learn how to use pre-infusion on a Bezzera DUO MN espresso machine.
How To: Pre-Infusion on E61 Brew Group Espresso Machines
Pre-infusion is what allows a small amount of water through first, before beginning the shot. It allows this water to saturate the coffee and get the grounds equally wet. This can make the coffee extract more smoothly. Water will always take the path of least resistance. By letting the water saturate the grinds before brewing at 9 bars of pressure, this can help prevent channeling.