The unit senses the noise of barking dogs.
When activated the device releases a very loud ultrasonic noise, this noise is at a too high a frequency for most humans to hear but is alarming and uncomfortable to dogs.
The dog will learn to associate this noise with the barking and in most circumstances the barking will desist or be greatly reduced.

For most dogs, they are sensitive to frequency around 23kHz and the device emits fixed frequency of 22kHz to 25kHz.

Settings

Unlike similar products this device is suitable for both Indoor and Outdoor use. 
This device has sensitivity settings on it, which allow you to control how far away the bark can be picked up from.
Test button - This allows you to check the device is functioning by emitting an audible noise for you to hear.

Positioning

Coverage - an oval 25 metres long by 15 metres wide.
Position the device pointing towards the usual area the dog will be in. 
Aim to achieve line of sight with the dog.
Because ultrasonic sound is a high frequency it behaves more like light than sound - consequently fences and dense shrubbery may block ultrasonic sound and reduce the effectiveness of the device.
If there is a fence between the device and the target dog, place the device high and point it down towards the dog..