US market-wide circuit breakers trigger when the S&P 500 drops 7% (Level 1) or 13% (Level 2) from the prior close, each pausing trading for 15 minutes if before 3:25 p.m. A 20% drop (Level 3) closes the market for the day.
They exist to slow panics, not to stop them. Individual stocks have their own trading-halt rules. Futures have overnight limit-down levels that work similarly.
Example: in March 2020 the Level 1 breaker triggered four times in two weeks, each time pausing the market for 15 minutes after a 7% decline.
Related: trading-halt, volatility, index, sec