Weell, ok paraphrasing the bing book, the slide type carbs had a plunger assembly that let a predetermined combustible air/fuel ratio to enter the carb throat. THe CV carbs are similar but use a rotary valve instead, and the mix is varied by lever position.
Doesnt specifically say there are three positions, although you can feel detentes.
"Why BMW? You like to suffer? My sister, she has five kids, no husband. You come over?"