I am not sure exactly how they are doing it but some of the dog men I hunt with have thier dogs trained to come to the horn on the truck. I think it may be that they blow the horn when they come to feed the dogs and also using a shock collar while blowing the horn. Maybe some one else can clarify it though. They all call em beep beep dogs.
I use the tritronics , and the truck horn..they work wonders on hard headed dogs...get them used to the shock collars in the yard..then the tone button is all you will need....then substitute the truck horn for the tone button..it doesn'tmatter how old or hardheaded the tritronics will fix them....I got 1 that I can stop in the road and he will sit there till I get there...tritronics and garmins have saved our sport