Our natural tendency is to hold the breath or use stress induced breathing (short and shallow) while holding a posture, especially in a challenging pose. This creates stress and tension in the body. In yoga, we want to create a calm and relaxed body, breath and mind through the conscious use of the breath.
There are several ways to breathe in asanas, and these will differ with teacher and tradition. I like to emphasize different breathing techniques or pranayamas while holding different postures. Most importantly, find a breath that supports your goal/intention that you are invoking for the pose and/or yoga practice.
• Use Dirga pranayama in most postures. Focus on actively breathing into the chest in backbends and chest opening poses (pigeon, yoga mudra, warrior I, fish, bridge). For forward folding postures and belly down postures you can focus on just breathing into the belly (child, forward fold, cobra, boat)
• Use Ujjiaj pranayama when holding strength building postures to increase endurance and focus (downward dog, standing squat, warrior III, warrior I, sun salutations).
• Use Kapalabhati Pranayama when you want to intensify holding a pose and to strongly activate the prana in the body. Kapalabhati can also create a strong focus during an challenging pose to allow us to stay present with the posture.