When there is a $5\%$ decline in the S & P $500$ that occurs before $3$:$25$ p$.$m$.,$ trading is halted for $15$ minutes.

The idea is to prevent human traders from moving share prices quickly in competition with algorithmic trading.

A $13\%$ decline in the S & P $500$ halts trading for the remainder of the day.

A $20\%$ decline halts trading for the remainder of the day.