Until recently, you could only go through North America exactly one time one a 1world explorer. Now, however, you are allowed through one additional time provided you are transiting out of (or, I believe into) South America.

For example, I was recently on a 1world originating in Asia that included three stops in the U.S., and then proceeded down through Argentina, Chile and Brazil. From there, my next stop was Dubai. In the past, I would have had to take either LA, BA or IB directly from South America to Europe. Under the new rules, I could have also taken AA to Europe by connecting in Miami or New York (I presume this would have applied to any of their other international gateways that I could have reached directly from South America, such as DFW).

Now, I didn't go so far as to ask on this extra North America transit stop if I was required to take the next flight out, or was allowed a 24 hour window.