Normally, a chip with an active LOW output would send a HIGH until triggered, thus if the intent was to turn an LED on only when a specific input was triggered, an inverter (NOT gate) would be required, followed by an LED with a resistor and ground. In this demonstration, the LED is reversed and a positive voltage is applied where the ground was. This way, the float HIGH leaving a chip would be negated by the +5V coming across the LED. Once the chip goes LOW, current can flow through the LED.
