The tester works by connecting each wire to an output at one end and an LED at the other ead. The outputs are switched on one at a time in sequence so that a correctly wired lead will make each LED light in turn. The 4017 IC counts up to 10 so there is a pause (for the 9th and 10th counts) before the LED sequence repeats. If the LEDs light up in the wrong sequence one (or both) of the connectors is wired wrongly. If and LED fails to light it indicateds a broken connection. Please note that the RJ45 computer network plugs cannot normally be re-wired, instead they must be cut off and replaced.