Even if you aren’t an electronics person, you probably have a working knowledge of circuit breakers. When the lights go out, you find the circuit breaker and flip it back to the on position.

The trading curbs go into effect when the S&P 500 crosses certain thresholds during extreme market volatility.

Here's what to know about so-called “circuit breakers" and when they're used to halt trading on stock markets.

Stocks fell sharply again Thursday, with declines nearing levels that would trigger a marketwide trading halt. These are the ...