Phototriacs can be zero crossing and non-zero crossing. This simply describes when the output current turns on. In a non-zero crossing phototriac, the output current turns on when the infrared emitting diode turns on, regardless of the AC voltage phase. In the diagram, the yellow line is the input current and the fuscia line is the output current. Note that when the input current goes high (on), the output current turns on immediately. For a zero crossing phototriac, the output turns on when the infrared diode turns on and when the AC voltage crosses zero. In the diagram, when the yellow input current goes high the output does not immediately turn on. Only when the AC voltage crosses zero does the output go high. Both turn off when the infrared emitter turns off and the AC line crosses zero.