When there’s enough resistance in the system to cause pressure to be greater than zero, changes to pressure will affect flow and changes to flow will affect pressure.

When pulling a shot, this state is reached when the puck is saturated and the basket is filled with water. The puck resists the flow, causing pressure to rise and flow to decrease. As water passes through the puck, propelled by the pressure, the puck becomes more permeable and the flow increases. If you have a pump with a bypass valve, it will hold the pressure constant at the preset value. Same pressure, less resistance, and greater flow. If you have a pump without a bypass valve, pressure will decrease as the flow through the puck increases. Less pressure, less resistance, and greater flow, but not as much as when pressure is held constant.

For the most part, when you see “flow control” being discussed, people are talking about slowing the flow during preinfusion. When you see “pressure profiling” being discussed, people are talking about changing the pressure and flow as pressure is rising in the basket and/or after peak pressure has been reached.

A modest amount of flow reduction as the machine ramps to full pressure can help prevent the puck from breaking up, which it might do if slammed by a powerful flow of water. Many manufacturers put a flow restrictor, or “gicleur” between the boiler and group to reduce the flow during preinfusion.

Further reduction in flow during preinfusion can aid in extracting light-roasted coffees. Often these coffees are difficult to extract, and if you try to grind them really fine to get more extraction you’ll choke the machine. But if you grind really fine and slow the flow rate during preinfusion, the water has time to gently open up the puck before it gets compressed by full pressure. This lets enough water pass through the puck to avoid choking the machine. Typically, these shots run longer than traditional shots, so extraction is aided by grinding finer and longer contact time with the hot water.

After the basket becomes pressurized, you can regulate pressure or flow. Changing either will affect the other. A common practice with light roasts is to reduce pressure and flow after maximum pressure has been reached in order to slow down the natural increase in flow rate that occurs as the puck becomes more permeable. This increases contact time with the hot water, potentially increasing extraction. You can do this with a variable speed pump or a valve.