✔ 最佳答案
HKMA issue Exchange Fund Bills (EFB) to capture (or to buy) HK$ in order to decrease the money supply of HK$ in the market. On the contrary, HKMA can redeem EFB to release (or to sell) HK$ in order to increase the money supply of HK$ in the market too.
You can see the issuance of EFB from the HKMA web site.