What dose the diaphram on the right side of the carb do?

 

I might have left this a little unclear, On the side that there is a plastic cap ( I have only seen this on a wolverine) it is a passage for the vaccum on the slide. When you unscrew this cap there is a spring in the middle to top of the plastic piece, behide the spring is the diaphram. A friend told me it controled the air to fuel/mix screw and jet. I just need to know if this is true.

 

it might be like a little accelerator pump