Reminds me of having to change the setting for her that determines how much time is valid between clicks to register a double-click. Self-clocking allows even my sweet grandma the ability to use her card without breaking her wrist. This make sense when you think about card swipes: everyone swipes at a different pace, some slower, some faster than others. It means you don't have to worry about looking for a clocking signal and timing the signal to center directly on the clock pulse, and no bothersome oscillating into the sweet spot of the clock signal.
Magnetic card readers are "self-clocking," meaning that they provide a clock called a strobe, against which the connected microcontroller can sync.