I've used wspan for more than two decades and worked in airline reservations prior to that. Learned how to overbook my first month at the airlines. However, since the airlines came up with married segment logic to defeat it (thanks wspan), it is much more difficult to accomplish on wspn.

Airlines will make certain classes available or not, due to yield management and fares offered in specific markets.
For example, Q might be sold out LGA-LAX, but available when you pull up LGA-LAX-SYD. Or L class might be booked up ATL-DEN, but magically appears when you pull up availability RIO-ATL-DEN.

Used to be no problem to just book it, then cancel the unwanted segment before ending transaction to retain the segment in the class of service you needed. Nowadays, joined segment indicators are appended to the connections due to the married segment logic inventory control which prevents just one of them from being canceled.

I can still get around it sometimes with triple segments, but it is a lot of work.
Non participanting airlines to married segment logic may be easier, but I don't book them much.

If there is a better way, maybe someone will stick out their neck and elaborate here.

Anyway, those were the days. Used to book further points, back to backs with impunity. Some airlines used to accept the long sell "IS" segment action status (if not holding, sold) and sometimes I could overbook this way. I used to long sell a sold out seat with the "LL" indicator (waitlist), then pull the PNR back up and "HK" the seat. With some airlines, particularly USAir, it would generate a message to in effect overbook that class, and I could then request a seat assignment and ticket. Doesn't work anymore and the discount fare classes now inhibit waitlists for the most part.