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A simple device that sits between dog collar and lead, that when pulled beyond a pre set tension, emits a high pitched whistle that dog hears but you can't. Annoys your dog so much he stops pulling, or rather 'gently reminds' your pooch when he pulls too much (got to beat a kick up the jacksy).
I suppose construction would be like those scales that fishermen use. Amazingly simple to make and cheap to sell. I'd pay a fiver for one anyway.