How can a central bank use direct intervention to change the value of a currency? Explain why

Question:

How can a central bank use direct intervention to change the value of a currency? Explain why a central bank may desire to smooth movements of its currency.

Exchange Rate
The value of one currency for the purpose of conversion to another. Exchange Rate means on any day, for purposes of determining the Dollar Equivalent of any currency other than Dollars, the rate at which such currency may be exchanged into Dollars...