An amp is slaved when you take the preamp and run it into the poweramp of another. That's the gist of it. The slave output is related to the speaker out somehow, that's what it says in the mesa boogie manuals.
Peavey 5150
Mesa Mark IV
Mesa Single Rectifier (Series 1)
Fender Custom Shop Tonemaster
Roland Microcube

-Whitebox OS 1x12
-Port City OS 1x12

Digidesign Eleven RackAxe Fx Ultra