The fuse of US stocks, also known as the automatic stop mechanism, means that when the main indexes of the US stock market, such as the Dow Jones Industrial Average (DJIA) or the Standard & Poor's 500 Index, fall to a certain threshold in one day, the market will automatically suspend trading for a period of time. This mechanism aims to prevent panic selling and excessive volatility in order to maintain market stability.

The specific rules for the melting of US stocks are as follows:

1. When the Dow Jones Industrial Average falls by 7%, the market will trigger a primary fuse and suspend trading for 15 minutes.

2. When the Dow Jones Industrial Average falls to 13%, the market will trigger a secondary fuse and suspend trading for 15 minutes.

3. When the Dow Jones Industrial Average falls by 20%, the market will trigger a three-level fuse and stop trading for the rest of the day.

It should be noted that the US stock fuse mechanism only limits the decline, not the increase. When the market resumes trading, if the index continues to fall to a new fuse threshold, it may trigger fuse again.

The history of the fuse mechanism of US stocks can be traced back to 1987. At that time, the US stock market experienced a serious crash, and the Dow Jones Industrial Average fell by 22.6% in one day, which made many investors suffer huge losses. In order to prevent similar incidents from happening again, the Securities and Exchange Commission (SEC) and the Chicago Board Options Exchange (CBOE) introduced the fuse mechanism.

The fuse mechanism of US stocks has not been triggered many times in history. In March, 2020, due to the rapid spread of COVID-19 epidemic around the world and the high market panic, US stocks were blown up four times in 10 days, which became a special period in the history of US stocks.