Belly Breathing, also known as Kapalabhati or "Breath of Fire," is a well known and commonly used yogic breathing exercise. I feel strongly that if you were to do one breathing exercise this would be the one. It is important to do some slow, long, calming breaths afterwards to ground and balance the brain and body. This breathing exercise helps bring circulation to the lower back, digestive organs, and reproductive system.
Kapalabati translates to "skull shining." It is believed by some that this exercise would cleanse the energetic and physical body, leaving a sort of halo or "skull shining."
This exercise can feel a bit awkward because the breath is inverse to how we normally breath. I recommend placing your hand or hands on your belly to help guide the breath. Start by simply inhaling while expanding the belly into your hand and then on the exhale draw the belly button towards your spine. Both inhale and exhale are through the nose and the mouth remains closed and relaxed. I like to do a set of breaths at a slow pace, then at a bit of a quicker pace and then at the quickest pace I can manage. Then I take a minute of slow calming breaths to restore equilibrium and will occasionally do a few sets.
Breathing exercises like this have a cleansing and oxygenating affect. If you feel dizzy or ungrounded for any reason please stop and lay down and focus on slow calm breaths. This breath has a heating and energizing affect on the body and is best when paired or alternated with a cooling or calming breath work exercise.