No, all it does is switch off the pump whilst keeping the exhaust valve closed. The system is still pressurised because the pump has been running and that pressure takes time to decline due to the resistance at the puck. Pressure then declines naturally as the residual water (espresso) exits the basket, as it has nowhere else to go until you drop the lever all the way and the exhaust opens into the drip tray. Dropping the lever immediately does no harm (that's what the designers intended) but it does mean that a lot more water ends up in the drip tray. Mine is tank fed with Waitrose Lockhills or Volvic.

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