I don't know the usage described, but makes sense in the context of the horse commands to me, in that, in computing for example one device usually acts as the 'master' and the other follows as the 'slave'.

> Google claims 28 hits but only shows nine, six of which are a repeat of a single citation in which it seems to mean "to laugh."

"gee haw" gets a few more (869), mainly to do with horses as far as I can tell. There was a 1928 cartoon book called: The Gee-Haw of the Modern Jehu!

And there's the Gee-Haw-Whammy-Diddle toy:-)
http://www.mugwumps.com/whammy.htm
Maybe that was once used to control horses?