SteveBell wrote:
..

Simply stand under the turbine with a stick of incense and watch the
smoke. Then stop the turbine and watch the smoke again. In the former
case, the smoke is sucked out quite rapidly; in the latter case, the
smoke goes nowhere.

A ridge vent - or hole in the roof - depends on a temperature
difference between the air in the attic and that outside to generate
convection currents.

A 12" wind turbine will move about 350 CFM of air in a 5MPH wind,
irrespective of the temperature differential.

With no wind, the turbine acts like a ridge vent, dependent entirely
on convection currents.

So what's the mechanism? How does the spinning top pull air out?

It's not a "spinning top." It's a fan.

A 12" wind turbine moves 350 CFM at a wind speed of 5mph, but it moves 1350
CFM at a wind speed of 15 mph. It should be obvious that, since its
effectiveness depends on the wind speed, it's more than a hole in the roof.

Here's the way I understand it works (and I could be wrong - the whole thing
may just be magic):

1. There's air in the turbine.
2. The wind spins the turbine.
3. The air in the turbine also spins is slung outward by centrifugal force.
The air exits the turbine thru the vanes.
4. This exiting air creates a vacuum in the space enclosed by the turbine.
5. The vacuum, in turn, sucks air from the underside (attic).

Both ridge vents and turbines have their relative advantages and
disadvantages. Turbines don't move much air when the wind doesn't blow;
ridge vents don't move much air in the winter.

In a light wind, turbines move considerably more air (about as much as a
robust bath exhaust fan) than a ridge vent

Both are cheap, both are passive, both trouble-free, and both use no energy.

I have ridge vents AND turbines on my house.