While this shares some of the mechanisms of a standard reduction of policy rates when they remain in positive territory, the adoption of negative rates provides additional accommodation by effectively relaxing the non-negative constraint on expectations of future short-term interest rates, thus helping to reduce long-term yields.
Let us briefly look at some figures: a)  as a result, the degree of real economic openness of many countries has increased significantly (as measured by total exports and imports of goods and services as a ratio of GDP).