Wayne:
Think in terms of a chain. Normally, your chain goes from the pick ups through the tone and volume controls in the guitar, then out the guitar's output jack to a cable, then to your effects, then out of the effects into an amp. Or, alternatively, guitar > cable > amp with effects loop and effects patched into said effects loop. For purposes of this discussion, let's assume the first model: guitar > effects > amp.
What Garcia did, and the standard Tribute and Further electronics provide, is effectively alter the chain to the following:
pickups > straight to effects > back to guitar, through guitar's tone and volume controls > out to amp. The purpose was for Garcia's effects to always see the same signal, not a signal that changed as the guitar's volume/tone controls changed. This helped him to get a consistent sound from his effects night after night.
To accomplish this, a stereo cable is used which is actually like squeezing two mono cables together. The signal goes from your pick ups out one side of the stereo cable to the input on your effects chain. The final output on your effects chain goes back through the stereo cable to the guitar where it now goes through the tone/volume controls, then out the mono cable to the amp. I believe the Tribute/Further electronics also allow you to put the effects loop after the guitar's volume/tone controls which then allows you to completely bypass the effects loop when not in use. I hope I got this right and it helps.
Bill, tgo