All electric fences have two parts, an active part and an inactive part. The active part consists of the fence charger's positive terminal, the hookup wire connecting this terminal to the conductor (wire, polywire) on the fence, and the actively charged wire or polywire that runs along the fence. If the fence has a ground rod (the usual arrangement), then the inactive or "ground" part of the system consists of the negative terminal on the charger, a wire connecting this negative terminal to a ground rod, the ground rod itself, and moisture in the soil running from the ground rod over to the feet of a target animal.
The target animal gets a shock when it provides a unique bridge between these two systems. That is, when it touches the active wire a charge passes from the active wire through the animal's body, through its feet, and out its feet into the water in the ground (dry soil cannot carry a charge but water can). Using this water, the charge travels over to the ground rod, up the ground rod, along the ground rod wire, and over to the negative terminal on the charger, thereby completing the circuit.
But suppose the ground is so parched or hard-frozen that there is not enough moisture available to carry the charge. Then the animal will not get a shock from the electric fence, because there is nothing to carry the charge from its feet over to the ground rod. In that case something must be done to remedy the situation or the electric fence won't work. One's first impulse is to get a more powerful charger or perhaps increase the number of ground rods. However, these are not ideal remedies. They may improve matters, but they don't get to the heart of the trouble. The best answer, and the only one if the trouble is really bad, is to replace the inadequate soil moisture with something else.
Suppose, for example, that you string another conductor on the fence that is a few inches away from some charged conductor. This conductor, however, is not charged, because it is not connected to the charger's positive terminal. Instead it is connected to the charger's negative (ground) terminal, and things are arranged so that it does not touch any charged conductor. Then, when the animal comes along and touches the active and inactive wires at the same time it gets a shock—with the charge passing from the active wire through the animal to the negative wire and over to the charger's negative terminal, thereby completing the circuit.