It is a time-synchronous keypad machine. The pinsentry takes the shared secret (pin) and the time and encrypts it providing a magic number (8 digit number). The Barclays bank computer, using a very clever piece of software, algorithm, de-crypts the magic number and bingo, you have access to your bank account.(There is a specific time frame for the magic number to be entered into you internet banking system. If the time is exceeded you cannot gain entry and so need another magic number from the pin sentry). Simple really!!!