autolite 2150/2100 booster assembly ???

working on a 2bbl autolite in a 77 ford right now. trying to figure out the funcion of the spring/piston/rod assemblly that attatches to the boosters. never came across this before since i don't touch newer (to me) fords. thought it was an anti pullover for the boosters but it doesn't seem to be on the same circuit. can anybody help me?? sorry for the blurry pic, only have my camera phone at the shop

 

It's a variable high-speed bleed system.

http://home.earthlink.net/~bubbaf250/2150carb/carb02b.html