Essentially inside the group head, there is a silicone ring which only lets water past with a small amount of pressure. When you turn the pump off, whatever pressure has built up will still be released into the coffee (we are talking like half a gram of water maybe) and at that point, water neither can neither exit the machine from the front or the back. If you want to flush all the water out entirely, you can put a cup below and the pump will blow it all out with air pressure.
Essentially inside the group head, there is a silicone ring which only lets water past with a small amount of pressure. When you turn the pump off, whatever pressure has built up will still be released into the coffee (we are talking like half a gram of water maybe) and at that point, water neither can neither exit the machine from the front or the back. If you want to flush all the water out entirely, you can put a cup below and the pump will blow it all out with air pressure.