The synchronizing of physical movement with your breath in hatha yoga is called vinyasa. This technique allows you to create a deeper absorption of awareness through a rhythmic moving meditation while practicing yoga.
Vinyasa also supports moving into and out of the asanas through connection with muscular breathing mechanisms. An inhalation naturally expands the belly and ribs and, with proper alignment, will lengthen the spine and encourage expansion. An exhalation naturally contracts the abdomen and torso and encourages retraction. In general, you inhale when you move into a pose, move against gravity, create upward movements or arch the spine. You exhale as you move out of a posture, move with gravity, create downward movements or round the spine.
Vinyasa is best used in warmups and during sun salutation sequences. Some styles of yoga use vinyasa through the entire practice, which can often sacrifice proper alignment in favor of moving quickly and fluidly through poses. However, the repetition of poses and movement with the breath does build familiarity, create mastery, build strength and increase body heat.
At first, the synchronizing of movement with your breath in vinyasa can be a bit discombobulating. However, once you get the hang of it and establish a rhythm that works for your body, vinyasa can ultimately deepen your focus and awareness.
Always remember not to sacrifice proper alignment to quickly move through a sequence. Slow down and take extra breaths if necessary. Vinyasa movement is best supported with the Ujjayi Pranayama breathing.