These draw air OUT, as the vent spins by the slightest breeze.
To install, remove the top cap and undo the spindle nut. The entire rotating section will remove and leave the base plate exposed. The base plate is screwed or riveted thru the 4 holes provided, and silicone or caulking compound should be added to the surface to prevent water leaks.
An optional internal cut-off vent may be added if required. Search for ‘muller round’.
The second image to the right shows the base added to the roof, then the rotary vent added. (Note: the retaining nut and cover cap have been left off in this photo)