It's official: China's central bank was behind the Chinese yuan's recent depreciation.

According to the latest data from the People's Bank of China, the central bank bought about 174 billion yuan worth of dollars in March, a purchase it made to drive down the value of the yuan against the U.S. dollar. U.S. Treasury officials have complained that the intervention by China's central bank was intended to keep the yuan undervalued. A cheaper yuan makes the nation's exports more competitive in foreign markets.

Big as the March figure is, the amount actually is lower than historical norms. For instance, the PBOC bought, on average, some 230 billion yuan worth of dollars a month last year to hold the yuan down, according to an analysis by Bank of America Merrill Lynch.

But that doesn't mean the Chinese central bank has stepped back from strong intervention in the currency market, as promised in recent months. Rather, it only suggests that the PBOC is taking a new tack to put further downward pressure on the yuan.

Normally, the central bank takes a step after intervention called sterilization. When the bank intervenes, it buys dollars from exporters, giving them yuan. Without action, that would expand the domestic monetary supply, increasing inflation. Because the bank wants to keep prices under control, it normally issues yuan bonds to soak up that excess liquidity.