Switches and red LEDs.
The slide switches (SWs) and red LEDs are the focal point and most interesting part of the DE2 board and Karibu.
The LEDs follow rules simmilar to the wireworld algorithm. If an LED lights, it propagates the light to neighboring LEDs. After being lit for one clock cycle, the LED is 'burned out' and can not be lit the following clock cycle, regardless of neighboring activity, ensuring unidirectional propagation.
If a switch is 'on', the corresponding LED is always forced on. LEDs neighboring forced-on LEDs can stay lit for two clock cycles. This creates a reflection effect when a 'wave' collides with a forced-on LED.
The video on the overview page shows this in action.