BEIJING, June 29 -- China's central bank injected 300 billion yuan into the financial system on Monday and deliberately said nothing about what it charged to do so. That silence was not an omission. It was the policy.

The People's Bank of China conducted its first-ever overnight reverse repo operation, a new short-term liquidity instrument the bank had flagged earlier in June. The operation injected roughly $44 billion into the banking system. The rate charged was 1.25%, according to sources cited by Reuters. The PBOC's official statement confirmed the operation and its size. It did not mention the price.

Analysts had expected the bank to price the new instrument between 1.25% and 1.35%. It came in at the floor. Becky Liu, head of Great...