1.) Mentally keeps you focused and in control. Knowing that you have to focus on your breathing gives you a task and a planned way to ride out your contractions.
2.) Physically reduces fatigue to the muscles in your body.
A good cleansing breath at the end of a contraction also helps you to relax your body and rest between contractions.
Breathing-Such a simple tool for labor and delivery.
To practice: Anytime you are going through something uncomfortable. Gas, period, stubbed your toe, bad bowel movement, pelvic exam, all these things will afford you the opportunity to breath deep and slow.
Look at it as an experiment to see if it works for you!