While priorities may differ between styles and teachers, when to inhale and exhale during asana is a fairly standardized practice element. Here are three simple guidelines for pairing breath with types of poses.
When bending forward, exhale.
When you exhale, the lungs empty, making the torso more compact, so there is less physical mass between your upper and lower body as they move toward each other. The heart rate also slows on the exhalation, making it less activating than an inhalation and inducing a relaxation response. Since forward bends are typically quieting postures, this breathing rule enhances the energetic effects of the pose and the depth of the fold.
When lifting or opening the chest, inhale.
In a heart-opening backbend, for instance, you increase the space in your chest cavity, giving the lungs, rib cage, and diaphragm more room to fill with air. And heart rate speeds up on an inhalation, increasing alertness and pumping more blood to muscles. Plus, deep inhalation requires muscular effort that contributes to its activating effect. Poses that lift and open the chest are often the practice’s energizing components, so synchronizing them with inhalations takes optimum advantage of the breath’s effects on the body.
When twisting, exhale.

In twists, the inhalation accompanies the preparation phase of the pose (lengthening the spine, etc.), and the exhalation is paired with the twisting action. Posturally, that’s because as your lungs empty there’s more physical space available for your rib cage to rotate further. But twists are also touted for their detoxifying effects, and the exhalation is the breath’s cleansing mechanism for expelling CO2.