Surround back connection

[Agent_sm1th]

I have a 5.1 set-up goin thru a sony 5.1 amp. On some dvd's (Finding Nemo for 1) a light comes on saying "Surround back decoding" Seeing this i checked the back where sure enough there was a plug in the "preout" heading for "SURR BACK" 2 questions... what exactly does it mean by pre-out? An why is the speaker connection a single phono?!?

[back_ache]

pre-out is the signal that comes after the volume control circuitry before it goes in the amplifier circuitry and is used for driving another amplifer. It cannot be used to drive a speaker directly