The left side simulates a microcontroller output. Sometimes there is only one controller pin left, but you need two pins for e. g. two LEDs. In this case you can use this trick. You can switch the output pin between high (+3.3V), low (0V) and tristate (high impedance). Now you can switch on the red or the green light. If you change between high and low quite fast, you can even switch both LEDs on. And now a question for our experts: Why don't the LEDs light, if the right switch is open (tristate)?
