trigger_add: <zone>.<variable> | <level> | string/rise/fall/both | oneshot/reset/<reset level> :: <command>

This sets a trigger on a variable

1)
i.e. if it's set to fall, it will only fire if the variable's value falls past the trigger value
2)
this is to provide the option of hysteresis in a the trigger mechanism and prevent over-firing in a noisy system.