Late in the game, purchasing Rock Bands and sending them on international tours is one of the best ways to direct bursts of Tourism at specific civilizations. Hallyu makes Rock Bands even more useful by allowing them to choose any available Promotion when they're first purchased and gain levels. Although its primary purpose is to speed up your Cultural Victory progress, using bands with Religious Rock can potentially help you win a Religious Victory instead.
This policy affects only Rock Bands that are purchased or gain levels after it is activated - bands that were ready for a promotion (whether they were just purchased or put on an exceptionally good concert) before it was activated will have three randomly-selected promotions to choose from instead of the entire pool.
Hallyu is the term for the “Korean Wave” of music and entertainment that has swept through Asia and the rest of the world since the end of the Twentieth Century. The success of this culture is credited to the efficacy of South Korea’s entertainment industry in spreading music (such as K-pop) and TV dramas (usually lumped together as K-drama). The term is occasionally used to describe any explosion of a nation’s cultural output across the world.