A dog whistle works by emitting a high frequency tone in a range that a dog does not ordinarily encounter.
Through this training, many dogs will develop a conditioned response to respond in accordance with its training when it hears the tone.
The highest frequency a normal middle aged human can hear is 12,000-14,000 Hz, so don’t be alarmed if you can’t hear all of the tones yourself.
Please note that dogs may not have an intrinsic response to high frequency tones such as those used in this application.
One method is to adjust the frequency of the tone, while your dog is sleeping, until your dog’s ears perk up.