Actually I believe he is still letting the two leads connected to the main circuit board connect to the wire feed motor. Then I believe he has one of the leads from the second transformer’s primary side spliced into the trigger relay. The two leads from the second transformer’s secondary side appears to be connected to the circuit board. That leaves one lead from the second transformer primary connection. This seems to be the mystery lead.
Obviously I could be all wrong about theses connection points. He seem to show his handy work as if everyone knew what he had done instead of teaching, but following him along, the above diagram appears to be correct.
If anyone knows for sure, we would all like to know.