It’s a very simple promotion at the core. Spend money for cash shop currency now while the game is in beta, and you can spend it freely within the cash shop in this test phase. Since the servers will be wiped, all of that money will be refunded to you after this test, but 15% extra on top of what you would normally get. So you get the money back and more besides, which is a pretty good deal if there’s stuff you already know you want to buy when the game goes live.
Source: Official Site