"In Hong Kong, most people use a contactless smart card called an "Octopus card" to pay for everything from transit, to parking, and even retail purchases. It's pretty handy: Just wave your tentacular card over the sensor and make your way to the platform.
But no one used their Octopus card to get around Hong Kong during the protests. The risk was that a government could view the central database of Octopus transactions to unmask these democratic ne'er-do-wells. Traveling downtown during the height of the protests? You could get put on a list, even if you just happened to be in the area.
So the savvy subversives turned to cash instead. Normally, the lines for the single-ticket machines that accept cash are populated only by a few confused tourists, while locals whiz through the turnstiles with their fintech wizardry."