Pull down to open? To open WHAT?

[Wade Nelson]

Knob on the dash below my ignitions switch, little do-hickey that sez "pull down to open."

to open WHAT?

[Wade Nelson]

from another thread:

It's the vacuum control knob. When the A/C is on or in the vent position, when it's open you have 85% air flow to the upper vents and 15% to the floor and when it's closed you have 100% to the upper vents and none to the floor. When the heater is on it has no effect.