Federal Reserve Board Governor Lael Brainard gave a speech on the U.S. economy in Chicago on Monday, saying that the Federal Reserve should remain cautious and should not raise interest rates too quickly, which aroused great concern in the market. She is also a voting member of the Federal Open Market Committee (FOMC), the Federal Reserve's interest rate decision-making body.

Brainard has long been viewed as one of the most dovish Fed officials. Michael Gapen, chief U.S. economist at Barclays Bank, believes Brainard's speech is a major signal about the Fed's intentions in September. If Brainard's speech sends a signal to raise interest rates, it will show a unified view within the Fed on raising interest rates.

According to CNBC, Brainard said that taking into account the still low inflation and the uncertainty of future economic development, the Fed has taken a cautious approach in exiting its easing policy. She believes this approach has paid off in recent months, helping to support continued employment growth and progress against inflation.

She also mentioned that slower economic growth and lower inflation expectations have become the "new normal." She said that although the U.S. unemployment rate is only 4.9%, wages are rising slowly and there is still room for improvement in the job market.

After Brainard's speech, the federal funds rate futures market showed that traders expected the probability of a rate hike in September to drop to 15%, from 21% before the speech. However, the probability that interest rates may be raised before the end of the year has increased, from 57.7% to 59.2%.

Last Friday, hawkish Fed Governor Daniel Tarullo and Boston Fed President Eric Rosengren gave speeches saying that the Fed was prepared to raise interest rates. The Dow Jones index fell about 400 points that day, or 2.13%.

But after Brainard's speech, the U.S. stock market sharply reversed its original trend. On Monday, the Dow Jones index rose 240 points, or 1.3%.

According to the "Fortune" website, Brainard's speech received close attention partly because she was the last Fed official to give a public speech before the Fed's September 21 meeting, and starting tomorrow the Fed will enter a week-long "silent period" before the meeting. On the other hand, it is because there are rumors that Brainard supports Hillary, and if Hillary wins the election, she may be selected as Treasury Secretary.

Brainard believes that after the financial crisis, the U.S. economic structure has changed, so it is necessary to re-evaluate the formulation of effective monetary policy. For example, because the relationship between unemployment and inflation has weakened, Brainard believes that although the unemployment rate is relatively low, this should not cause the market to worry that inflation is coming. Instead, the Fed should focus on hidden weakness in the job market, such as too many employees working part-time but in desperate need of full-time positions.

In addition, Brainard said the market has to get used to the possibility that interest rates will remain low for an extended period of time. She said the past seven years have proven that low interest rates are necessary to maintain solid economic growth and combat inflation.

"Fortune" analyzed that last week's speech by a hawkish official from the Federal Reserve caused the stock market to fall sharply, and this time the market responded so enthusiastically to Brainard's speech, indicating that the change in the possibility of raising interest rates in September caught investors off guard. But when it comes to when the Fed could raise interest rates at the earliest, there have been wide divergences in views expressed by several Fed officials in recent weeks, which can be confusing.

As David Kelly, chief global analyst at J.P. Morgan Funds, wrote on Monday, traders will have to judge whether "the more hawkish views expressed by some Fed officials mean that the Fed is actually considering raising interest rates or is just faking it."

Brainard's speech on Monday was actually a reminder to the outside world not to take the September interest rate hike too seriously. However, on the other hand, the FOMC released members' expectations for interest rate hikes in June, with most members believing that there may be two interest rate hikes this year. Considering there are only three meetings left this year, it may also mean a rate hike next week.