The best part is the advice that he is giving to people in his comment section.
"The yellow line is just for reclaiming the refridgerant in the system already or for putting refridgerant in the system.
Aside from that, when your just checking the pressure it has no use. Just make sure its capped off or running to something. Otherwise when you open your valves on the gauge it'll just come out the yellow line and vent to the atmosphere."