How's this example?

Let's say you have an inlet opening of a given area
and downstream of that is a constant area duct at the
same area. At the exit of the duct is a door flap that
can be set to anything from fully open to fully
closed.

When the door is fully open you will get the max flow
through the duct. It might be 99%+ of the freestream
flow at that same cross-sectional area.

When the door is fully closed you get zero flow
through the duct. All flow streamlines steer right
around the inlet opening. This is a pitot tube
basically.

When the door is set to an intermediate position, you
can get any flow you want through the duct (between
zero and max) and the inlet area is the same.

Changing door positions is like swapping in and out
different radiators and ductwork that have different
flow resistances. When going to a higher resistance
radiator (for instance, one with smaller frontal area,
a thicker core, and the same fin spacing) it is like
closing the door a bit. The air flow will decrease.

So, my point is that you can get any flow from zero to
max flow through an inlet by changing the downstream
flow losses.

Ron