When wrestlers from different promotions appear on each other's shows. For decades, WWE operated as if no other wrestling company existed. AEW changed that by actively collaborating with NJPW, AAA, and others. The 'Forbidden Door' became a literal pay-per-view name. Now even WWE has opened up slightly with cross-promotional appearances. The concept that talent can freely work across companies was genuinely revolutionary after thirty years of WWE's walled garden.
"He showed up on Dynamite even though he's under WWE contract. The forbidden door is wide open."