Can someone explain what controls the choke on the driver-side barrel of a 22r carb?

I just bought an '88 PU and when the weather dropped it became harder and harder to start. When I looked in the carb I discovered that the choke was disconnected and a few parts were missing. Now I'm trying to repair it with spare parts from an old 22r carb.

I understand how the primary choke butterfly opens and closes, but seems like the secondary butterfly is only connected the throttle. Can anyone help?