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Knluk 1/ while keeping the flow control closed, the pressure will build up slowly after I press the button. Then preinfusion starts.
The paddle for the flow control can be adjusted so that in the closed position you can have no flow at all, or that you have some flow (e.g. a trickle). You can easily check by lifting the lever without a portafilter and seeing if any water is coming out of the group.
If in the fully closed position your valve is completely closed (no flow at all), then if the paddle is in the fully closed position there will be no water entering the group and no pressure will build up (so no pre-infusion).
As for changing the pressure with the flow control - once the group is under pressure during a shot there will be water going in to the group and water coming out (through the coffee). As long as the flow into the group is the same or higher than the flow out through the coffee, the pressure will be maintained. As soon as the flow in becomes slower than the flow out (by closing the paddle), the pressure will gradually reduce. This is how you can change the pressure by closing the valve to slow the flow of water into the group.