POS on is used when there is a PARENT OVER SHOULDER watching what u say and it is a warning sign to your friend online that they should watch what they say.
POS off is used when there is no longer a parent over your shoulder.
Mark: wtf are you talking bout?
Mary: POS ON
Mark: yeah i did watch the new episode of family guy :)
Mary: yeah it was hilarious
Mary: POS OFF
Mark: okay anyways..back to topic..wtf were u thinking?