During the second stage of labour, your baby moves deep down into your pelvis. His chin is pushed firmly down on his chest and his head turns so that the widest part (from front to back) is in the widest part of your pelvis (from your pubic bone to your tail bone).
When his head is being born, you feel a hot burning sensation between your legs as the perineum (the tissue which extends from the back of your vagina to the back passage) stretches. Then your baby's head turns to face your inner thigh in order to bring it back into line with his shoulders which are twisting their way through the pelvis. Your baby's shoulders and body are generally born in a single contraction after the birth of his head.
As you push, you feel strong contractions deep down inside you and a lot of pressure between your legs. You may feel as if you want to open your bowels. This is entirely normal and your midwife will hold a soft pad against your back passage in case a small amount of faeces escapes. You will find that when you think you're almost there, the head slips back between contractions. This "two steps forward, two steps back" process it vital to allow the tissues and skin to stretch slowly. Try not to tense up because this will stop the tissues stretching and you may be more likely to tear.
Your instinct will be to push, perhaps holding your breath, in short sharp bursts. You won't want to push non-stop from the beginning of each contraction to the end. Nature intends you to push only briefly because your baby's oxygen is diminished while you are pushing and you need to keep his supply going by breathing regularly during each contraction. Some women don't hold their breath at all while they are pushing. Their breath escapes in grunting noises.