what is the 'economy' mode exactly?

[ZiggyPA]

I am wondering what the 'economy' mode on the climate control is supposed to do as opposed to 'auto' and 'heater' ?
Today I had the airco on auto on a long trip, and tried the 'economy' button. The air from the vents becomes musty and lukewarm and I swear at some point I saw some vapor, smoke or dust coming out of one of the vents
I put it on 'auto' again and the air gets cool and fresh again...

[maxwedge]

No compressor engagement, basically a vent.