They explicitly say that the signals are audio and vibration, not electric shock.

The signals are audio and vibration Backed UP by an electric shock.

  CHARLIE BAKER: Cows learn that a sound cue and increasing beat is a warning that they're approaching a virtual boundary, and they learn to turn away from that boundary.

  In the first few days of training, if they ignore that sound cue, they would get a low-energy pulse. That's significantly weaker than an electric fence, but it's enough to give meaning to that sound cue.