Hello Norm,
You have it right!
I just took two computer fans rated at 12v 280ma to run an experiment:
Connected to 12v battery, current 263ma. Unrestricted air flow in and out.
Blocked the intake, by laying it down on the table. Current 253ma.
Flipped it over, 274ma, could feel air blowing back out the input, so air was still moving.
The current should have gone down. These fans are not tight, air can get around the blade tips.
Put a box on it Blocking both input and output,. Current 217ma. Validation of above.
The fan speed up, if friction loss were greater, the fan would not speed up.
Connected two fans in series, current 109ma. fans slowed down.
I didn't have any way to measure air flow or RPM. Lower air flow requires less current.
Power is in the moving of the air. Restricting the input or output should reduce the power. Air friction does draw power, but not as much as moving the air.
Blocking the input or output should have the same effect as blocking both in a tight fan. (no air around the tips).
Not as efficiently as slowing down the blade.
Most Car engine Fans are designed to be PWM speed controlled. Lowering the voltage will do the same, doing so efficiently may be a problem.
Wire two fans in series, both MUST be the same motor so they share the current equally. The current dropped more than half in the above experiment. The air flow is reduced, but You have two fans running, using less current.
I run fans in my computer off 7v or series all the time to reduce the current. With 22 computers it gets costly for the power.
Have fun,
Scott.