Push fluid through a narrowing pipe — it speeds up and its pressure drops. The same volume per second must squeeze through the throat, so it accelerates; and where fluid moves faster, it pushes sideways less. The manometer tubes read the local pressure: watch the throat's tube sink.
• Flow particles▮ Manometer pressureHorizontal Venturi · same height throughout
Try this: shrink the throat and watch its speed rocket up while its manometer drops. Squeeze it enough and the throat pressure falls well below the inlet — that suction is the Venturi / aspirator effect used in carburettors and spray pumps.