boshk - as @DavecUK says: pull on the lever and hold it there until the pressure shows zero, and remove the portafilter. Obviously, after the pre-infusion stage. This will essentially move the piston up, stopping the pressure on the water inside the cylinder. But… even though Dave says no drama and no fuss, you still have a cylinder with 100ml of water (or whatever the capacity is) which needs to come out. So be prepared for a few splashes, thankfully, at atmospheric pressure. 😉
Edit: how do I know? Same with a La Pavoni, just the opposite direction with the lever. 😉
