Not in every case a diffusor increases the pressure in a fluid. Take a water reservoir and make a lot of small holes into the ground. The water will go through the holes, the water level inside the reservoir and by this the potential energy and the the pressure decreases.
Now take a long pipe which is connected to the ground of the reservoir and let water going out through this pipe. The water will have some velocity and by this some kinetic energy. Now decries the cross section of the valve and for a moment the pressure inside the pipe increases. For a long pipe it is somehow forbidden to use fast closing valves because a hydraulic knock appears and could destroy the pipe.
Now imagine that your water reservoir is below the level you want to use the water and between your reservoir and the valve is a pump with an electric device. Open the valve, start the pump and water is flowing. Now make the cross section of the valve smaller and the constant power of the pump will increase the pressure in the pipe.
In detail, why in the mentioned examples the decrease of a cross section increases the pressure? Take a meat grinder and remove the diffusor. The meat will go through faster (but not very well shredded). So the decrease of the cross section changes the possible flow of a fluid at this place and if some kinetic energy applied at the starting side stays unchanged the pressure increases.