Whazziss?

[capturedlive]

I recently purchased a 1972 GMC 2500 Sierra from the original owner. It's got the 402 cu. in. engine with 400 turbo transmission, and it's in pretty great shape.

Here's my question: Under the dash, behind the heater hose, there's a button that pushes up. The former owner told me it does something to add oomph to the throttle and that it came stock with the truck. I later looked through the manual, did Google searches and even talked to a couple of mechanics, and I'm not able to find anything on that gizmo. Does this sound familiar to anyone?

Thanks ahead of time.