Roger here's some answers to your mystery. Brian H explains active shielding partially, yet there's a bit more to it. Synergistic applies 15 or 30 volts DC to the cable shield, which of course draws a small DC current through it. The shield is of course discrete (not in the signal path). This shielding current helps to prevent the coupling of stray EMI & RFI into the signal conductors. I use a dual-module Synergistic Master Control Center which has both 15V & 30V outputs; experimentation can determine which shielding voltage works best with which cable; the results can vary significantly depending upon whether it is a signal cable or a power cable. The selection of AC cable supplying power to the Master Control Center also affects sonic characteristics to a significant degree. Synergistic's Ted Denney can explain all this to you in greater technical detail than you might ever desire to know.
The shielding voltage also accomplishes another function. It saturates the insulation dielectric, which if left to its' own will actually leach electrons (audible micro-information) away from the signal conductors.
Audible effects of active shielding can vary of course from one rig to another. In my rig, I perceive improved bass extension & control, smoother midrange vocals, a greater sense of depth, staging & HF detail. The greater that the capabilities of your equipment are, the more improvement that you'll hear with the active shielding system.
For economy applications, Synergistic also offers a mini power coupler (wall wart DC power supply) in lieu of the Master Control Center. I have never tried a mini power coupler so I cannot compare them for you, but I suppose that performance wouldn't be as good since you don't have the optional output voltage selection nor the AC power cord selection capability. Still, based upon my own experiences, I definitely recommend trying it out for yourself one way or the other. I have heard no downsides when using active shielded cables. Of course there is some additional expense however.