Pressure profiling involves varying the brew pressure throughout the shot. In its simplest form, the shot is divided into a number of stages, with a different pressure for each. Machines that offer this kind of pressure profiling include the Synesso MVP Hydra and the Sanremo Opera. Other machines, such as the La Marzocco Strada and the Decent Espresso machines, allow the barista to control the pressure dynamically throughout the shot.
Most commonly, pressure profiling is used to provide first a preinfusion stage and then a period of lower pressure towards the end of the shot, sometimes called post-infusion or ramp down. Towards the end of a shot, as more of the coffee gets extracted, the puck begins to break down, which can lead to channeling. Reduction of pressure at the end of the shot can mitigate this, potentially allowing higher and more even extractions.
When brewing at a fixed pressure, the flow rate also increases towards the end of the shot for the same reason (the puck begins to break down), which means that the time that the last few grams of water is in contact with the coffee is very brief. Reducing the pressure offsets this, increasing the contact time of the last few grams, which James Hoffmann (2011) speculates could increase the extraction per gram of liquid in the last part of the shot.
When using pressure profiling, most baristas choose to use a low-pressure preinfusion at the start of the shot, followed by an increase to full pressure and then a gradual reduction in pressure towards the end of the shot. By chance, this profile is similar to the pressure profile produced by a lever machine. In a lever machine, pulling down the lever opens a valve, which fills the brew chamber with water at line pressure. When the lever is released, the brew water gets pressurised by a spring in the group.