Pre-market has S&P down ~3%. I think we’re going to hit the 7% circuit breaker today

Historically when it comes to stock market crashes, a foreign market will have a smaller correction some time immediately before a larger and more serious correction in the United States.

How does the circuit breaker work? Does it trigger based on a timer or after each trade? Like, could the market crash so fast in the space of 5 seconds that it goes an extra % point or two below the magic number?