If it is a stock (or other security, including a bond), you can borrow it and agree to sell the stock to someone at some specified date in the future at a specified price.

Now consider the case in which the central bank wants to establish a bond price that is above the market equilibrium price; in fact, the central bank is then pursuing a minimum price policy for bonds.

With each new leg higher, the bond market gets more expensive and interest rates, which move in the opposite direction of price, fall even lower.