Output connectors transmit a funny digital test signal, and the input connectors detect this signal and display the results (of which pins on one end of the cable are attached to which pins on the other) on the LED matrix.

The matrix is very useful as it tells you which pin from the IN goes to where on the OUT.HOWEVER: When testing cables like a 3.5mm TRS (headphone) to stereo RCA, the unit's response can be confusing

The LED matrix looks identical to the one on the Behringer CT100 cable tester.

The LED matrix can only test three pins at a time.