Hold the bottle in a position and keep the nipple full of milk so that you can prevent your baby from swallowing air with the milk. (newbornbabyzone.com)
This angle also keeps the nipple full of milk, helping to reduce the chance of baby swallowing air. (chiccousa.com)
This is especially true if baby has swallowed some air while feeding or when crying. (health-and-parenting.com)