Wikipedia
PassWindow is a technique of producing one-time passwords and facilitating transaction verification that is used as an online second-factor authentication method.
The system works by encoding digits into a segment matrix similar to the seven-segment matrices used in digital displays. The matrix is then divided into two component patterns that reveal the whole when superimposed.
Half of the pattern is printed on a transparent region of a plastic card, while the other is displayed on an electronic screen such as a computer monitor. These are referred to as the key pattern and challenge pattern, respectively.
Each key pattern is unique, and the challenge pattern can only be decoded by its corresponding printed key.
By varying the challenge pattern displayed on the screen, a series of digits can be communicated to the card holder without being visually revealed on the screen.
PassWindow is typically implemented such that an animated, perpetually looping sequence of challenge patterns is displayed, each encoding a single digit placed in a random location within the matrix.
A valid solution to this challenge then consists of a specified number of consecutively appearing digits.