A surround-sound decoder that supports a central channel will pick out the identical signals in the A stream and B stream based on their pattern and amplitude. In a surround setup with no center speaker, the perfectly balanced center signals will create a "phantom speaker" (the illusion of a speaker) directly in between the left and right speakers.
The sound signal for the surround channel is also recorded on stream A and stream B, but the identical signals in each stream are out of phase with each other. Instead of playing in synchrony, they are shifted in time in both audio streams. The result is that the two signals work opposite one another: When the surround signal in stream A tells the left speaker cone to move out, the signal in stream B tells the right speaker cone to move in. Because of this, the surround signal information coming from the front left and front right speakers largely cancels itself out, and you don't hear it.
A surround-sound decoder receives both stream A and stream B and shifts them relative to one another so the surround signals are in phase again. With this shift, the right, left and center signals are all out of phase, and so tend to cancel each other out.
In addition to separating the different signals, proper surround decoders pass the audio information through different filters and noise-reduction elements to balance sound levels and reduce noise. Pro Logic decoders use active "steering" elements to control the process more precisely. Check out Dolby Surround Pro Logic Decoder: Principles Of Operation (PDF) for more information.
Lots of home audio hobbyists have figured out a way to partially unlock the surround channel using only a two-channel home stereo and an extra set of speakers. In the next section, we'll see how this bare-bones surround-sound setup works.