SUs have a piston that runs up and down in that cylinder above the barrel, that opens and closes the throat when vacuum from the section between the butterfly and the piston varies...
Strombergs operate using a diaphragm, but it's still functioning the same way.
I believe, however, that the strombergs don't have the needle type of operation that the SUs do. I don't have one here to look at so I can't confirm it, but I seem to remember reading somewhere that they have a screw adjuster in the base.
I'm sure you'll find diagrams and more online. The Strombergs are called 'CD' types.
