It's the practice of adding a (usually final) destination (on another carrier) with which the ticketing carrier has limited formal interaction. The lack of IT systems integration, plus 1960s technology, causes YQ to be zeroed out for the whole booking.

You want to go AAA-BBB, so you book AAA-BBB-CCC, where BBB-CCC is on some propeller charter company start-up flying bush tours. The airline operating AAA-BBB has a simple operating agreement with this startup, but no more. Your ticket is AAA-CCC so the YQ calculation doesn't sync with the existing systems and is set to 0.

It's hidden city ticketing plus a cancellation of YQ as a benefit. It's very hard to find these days, so it's of very limited use.