"They" don't do anything - this is called a circuit breaker, and is automatically triggered. There are three breakers:
L1 - 7% down before 3:25pm - 15 minute halt
L2 - 13% down before 3:25pm - 15 minute halt
L3 - 20% down - halted for the remainder of the day
Only a single L1 and a single L2 breaker can occur in a single day, e.g. the market falling below 7%, rising, then falling again will not trigger a second L1 breaker, but falling to 7%, up to 5%, then down to 13% would trigger an L2.
FYI this is the kind of thing you have to know to become registered as a securities representative.
L1 - 7% down before 3:25pm - 15 minute halt
L2 - 13% down before 3:25pm - 15 minute halt
L3 - 20% down - halted for the remainder of the day
Only a single L1 and a single L2 breaker can occur in a single day, e.g. the market falling below 7%, rising, then falling again will not trigger a second L1 breaker, but falling to 7%, up to 5%, then down to 13% would trigger an L2.
FYI this is the kind of thing you have to know to become registered as a securities representative.