How an Electric Fence Works
All electric fences have two parts, an active part and an inactive part. The active part consists of the positive terminal on the electric fence charger, the insulated electric fence wire connecting the electric fence charger's positive terminal to the charged wire or other conductor, and the actively charged wire that runs along the fence. The inactive or "neutral" part starts at the negative (ground) terminal on the electric fence charger. In the traditional setup using a ground rod, this inactive system includes a wire connecting the negative terminal to a ground rod, the ground rod itself, and moisture in the soil running from the ground rod over to the fence.
Your pet gets a shock when it unsuspectingly provides a bridge between these two systems. That is, when it touches the active wire a charge passes from the active wire through your dog'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 ground terminal on the charger, thereby completing the circuit.
But suppose there is no water in the ground, or the ground is hard frozen, or your dog's feet are off the ground when it touches the charged wire because the wire is at the top of the fence. Then the dog will not get a shock, because there is nothing to carry the charge from your dog's feet over to the ground rod and thence back to the charger.
In all these cases, something must be done to remedy the situation or the electric fence won't work. The best answer, and the only one if the trouble is really bad, is to provide another route for getting the charge back to the charger.
If you happen to be mounting the charged wire along or on a bare metal fence, you can use the metal fence to get back to the charger. Just use any available metal wire to connect the metal fence to the ground terminal on your charger, making sure that all parts of the metal fence are connected electrically to one another and that none of them are touching the charged wire. Then your dog will get a shock when it touches the charged wire and the metal fence at the same time, because the metal fence will carry the charge back to the charger.
You can do the same thing, if your existing fence is not bare metal, by placing another wire on the fence that is 4 to 6 inches away from the charged wire. This second wire is not charged, because it is not connected to the charger's positive terminal. Instead it is connected to the charger's negative terminal, and things are arranged so that it does not touch the charged wire at any point. Then, when your dog 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 your dog into the negative wire and over to the charger's negative terminal, thereby completing the circuit.