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It's a device for motivating horses to pull your carriage, which was high tech at one time but was obsoleted by the introduction of the motor car. It became a metaphor for technologies obsoleted by other advancements. The irony is that in the olden days, from my perspective, it would have been a massive pain in the ass to get one, because you'd probably have to mail a check to some Amish mail order company and wait a month, but now I can just click over to Amazon and get it in two days.