EE help or anyone familiar with electric fence energizers

[Zebo]

At the bottom of this page is model ZAREBA ADD4 which indicates a "Patented direct-discharge technology" energizer which means it only needs ONE wire and no ground. How does this work? I've always understood you needed two wires on top of the fence(one + and one - ), or one wire with a ground sunk into the earth which complete the circut when contact is made with the horse. This is the first energizer like this I've seen but could certainly save me some money by not haveing to pound in grounds every 100 yards or run dual wires. Thanks

 

[OS]

probably uses the Earth as the 'other wire.'

 

[tweakmm]

It's not called a ground for the hell of it