Handyman question - Flushing the Sprinklers

[swede123]

A cry for help from my fellow OOT homeowners.

So it's time to winterize the sprinklers which involves flushing the system with compressed air. I have a compressor and hoses that connect to the valve at the sprinkler system. However, the valve has two distinct plug-ins where to attach the air hose, and I don't know which one to use. There are no markings on the valve, simply two plug-ins, one above the other, both with 1/4" fittings. Any help would be appreciated.

Swede