The central bank cannot actually stem the tide in a loss of market confidence, central bank actions all depend on people retaining confidence in the currency to

The central bank cannot actually stem the tide in a loss of market confidence, central bank actions all depend on people retaining confidence in the currency to begin with. If a central bank wants to buy government bonds from all the sellers and government treasury, it is always purchasing by diluting the currency. Once the market gets tired of bagholding the currency then the central bank has outlived its utility and the government cannot sell any more bonds because all the sellers evaporated.