I've posted on this before, this is a total nonexistent benefit. I have tried to invoke it about a dozen times, on my own and thru a travel agent for WBC flights to/from Europe or to/from Asia. I have even tried calling ack 3 or 4 times. I think the last time I tried I was told at most they would make a reservation (wait list?) for me but there was no way I would actually get on the plane! They pretty much told me that when J is already oversold they will not honor this "guarantee" so I should make sure I had tickets on another carrier...
I'd love to hear from someone who successfully invoked this.