How the State Bank of Vietnam’s federal reserves are sold into the market

The State Bank of Vietnam–the SBV–has started selling its US dollar reserves in order to stop the local currency from devaluing further. According to this article from Tuoi Tre it is doing this by selling its dollars to banks that hold negative foreign currency positions. They are allowed to buy enough US dollars to bring their foreign currency balances back to zero. It’s not clear why it has been structured this way.