In a nutshell, there's a new kind of customs union called a hybrid customs union. When countries form this union, it can either create more trade or shift existing trade. The balance between creating new trade and diverting existing trade determines if the union benefits the member countries. If the boost in trade outweighs the redirected trade, everyone wins. Conversely, if more trade is diverted than created, countries might lose out. And if the two forces are in equilibrium, there might be no change in welfare. So, the success of a hybrid customs union hinges on whether it generates enough new trade to offset any trade shifts.