I found this information about using the Ujjayi breath during practice, on a Yogahub post and thought it was clearly written. If you want to read the whole article see the link below.
The ujjayi breath (ooh-JAH-yee) is translated as the victorious breath. It allows us to breathe in a controlled manner. It is a deep full sound that increases the movement of the diaphragm and lengthens the spine. The soft consistent sound aids the even flow of the breath and helps to build heat in the body, which is conducive to stretching and cleanses and purifies the body. Unlike other pranayamas that are generally practiced while seated or lying down, ujjayi is maintained throughout our yoga practice.
The ujjayi breath is the starting point for any asana practice. The steadiness, sound, and depth of the ujjayi breath help to link the mind, body, and spirit to the present moment. This unification adds richness and depth to your practice and helps to cultivate presence and awareness.
With consistent practice breathing lengthens throughout the practice, awareness of your body’s needs of movement increase and injury is unlikely. If breathing is soft and consistent it is almost impossible to push too hard. Always remember that breathing should never be at the expense of a posture Throughout your yoga practice, aim to maintain the length and smoothness of the breath as much as possible, endeavor to maintain the quality of breath throughout the practice.