Step 5: Tri-states (not tricycles)
Of course manually disconnecting the pins is a bit difficult to do, particularly if we are scanning things very quickly to use the persistance of vision effect to show a pattern. However a microcontroller output pins can also be programmed to be input pins as well.
When a micro pin is programmed to be an input, it goes into what is called 'high-impedence' or 'tri-state'. That is, it presents a very high resistance (of the order of megaohms, or millions of ohms) to the pin.
If there is a very high resistance (see diagram) then we can essentially regard the pin as being disconnected, and so the charliplex scheme works.
The second diagram shows the matrix pins for each combination possible to illuminate each of the 6 LEDs in our example. Typically a tri-state is denoted by an 'X', 5V is shown as a '1' (for logical 1) and 0V as a '0'. In the micro firmware for a '0' or '1' you'd program the pins to be an output and it's state is well defined. For tri-state you program it to be an input, and because it's an input we don't actually know what the state may be....hence the 'X' for unknown.
Although we might allocate a pin to be tri-state or an input, we don't need to read it. We just take advantage of the fact an input pin on a microcontroller is high impedence.