Explain the Working of MOC70PX - PHOTOTRANSISTOR OPTICAL INTERRUPTER SWITCH

hai all,

i am doing a project based on MOC70PX - PHOTOTRANSISTOR OPTICAL INTERRUPTER SWITCH for that i need to know the operation of the device...
my circiut is like this ....



please explain me the working of it as soon as possible...coz i have to give a seminar on it...in my college.

thanx in advance,

regards
teja