Okay. First of all lets discuss what is meant by a positive feed back mechanism. In simplistic terms, this is where the process of adding something will increase what is already happening. So what does Oxytocin do? Well if you are a woman in labor, you are experiencing a lot of pain because the walls of the uterus are contracting (essentially squeezing the baby out). Oxytocin plays the role of simulating more contractions and therefore by make more contractions more Oxytocin is released! its a continual cycle of increasing contractions and increasing the hormone levels to help push out the baby.
Does that make sense?