a method of conducting an economic transaction includes the steps of wirelessly receiving a card - transmitted signal , where the card - transmitted signal includes user account information and an indicator of an amount of elapsed time between a recognition of a vibration applied to a wirelessly - transmitting credit or debit card and a transmission of the card - transmitted signal from the card . the method further includes determining a last detected vibration at a point - of - sale device , calculating an amount of time elapsed since the identification of the last detected vibration , and comparing the amount of time elapsed since the identification of the last detected vibration with the amount of elapsed time between a recognition of a vibration applied to a card and a transmission of the card - transmitted signal from the card .