this is not a bad explanation from babycentre.com.
hardly anyone these days has a natural third stage in hospital, so they don't get the benefit of hte pulsating cord. there is a middle ground btw.
Whether you have your baby at home or in hospital, you can choose the way the placenta and membranes are delivered. A managed third stage means you have an injection in your thigh just as your baby is being born which causes the uterus to contract strongly and separate from the placenta. This allows the placenta to be delivered quickly. You do not have to push or do anything as the midwife will wait until your uterus is contracted, and then pull gently on the cord to deliver the placenta. The cord will be clamped soon after the baby is born. The advantage of this method is that it is over quickly, usually in five to 10 minutes, and there is little blood loss. The disadvantage is that the oxytocic drugs used may make you feel sick or faint, or give you a headache.
A physiological or natural third stage means that you wait for the placenta to be delivered naturally. This may take up to an hour to happen, though skin-to-skin contact and breastfeeding your baby will help the uterus to start contracting. You need to actively help the delivery of the placenta by pushing, perhaps in the squatting position. Cord clamping is left until after the cord has stopped pulsating, so that the baby gets more oxygenated blood from the placenta. There is usually more blood loss with a physiological third stage, so it is not advisable if you have had any complications of the pregnancy such as anaemia, heavy bleeding or high blood pressure, a twin pregnancy, an induced or very long labour, or an assisted delivery.
Discuss the options with your midwife, and make your wishes clear in your birth plan. If you choose a physiological third stage and it takes longer than you expect, or if you do start to bleed more heavily, your midwife can give you an injection to deliver the placenta at any time