Constricted Breathing Patterns

Before attempting to manipulate the breath with pranayama, it makes sense that we and our students would have a felt sense of what unhindered, natural breathing feels like. Different teachers refer to this as natural breathing, free breathing, belly breathing, yogic breathing, diaphragmatic breathing and essential breathing. Experts tell us that often people's unconscious breathing is not free but rather is restricted and incomplete.
Even with clear instructions, it takes many weeks of practice before some students can actually breathe fully all the way to the bottom of their lungs, and even longer for some to be able to rapidly pump their bellies toward their spines in an energizing exercise like Kapalabhati." – Amy Weintraub, Yoga for Depression: A Compassionate Guide to Relieve Suffering Through Yoga © 2004, pg. 127

Following are ways in which our students may be inhibiting their free breath.

Reverse Breathing
Some people* are "reverse" breathers which means the belly doesn't expand on the inhale but rather contracts & instead moves out on the exhalation (although not likely in a fully relaxed way).
This type of breathing is associated wtih chronic tension, digestive isssues, insomnia, and elevated blood pressure.
The cause may be related to tight clothing, disease or repeated feelings of fear.
*Mukunda Stiles makes the amazing comment that "about half the population" breathes in reverse. (Structural Yoga Therapy: Adapting to the Individual © 2000, pg 49)

Chest Breathing
In this case, the person chronically holds the abdomen in, forcing the breath to be experienced only up in the chest.
This typically results in chronic tension, digestive issues & anxiety.
The cause can be stress or holding the stomach in to appear thinner.
Unconscious Stress Breathing
While we haven't seen the exact phrase "unconscious stress breathing" in the expert's writing, we learn from them (and our own practice) that the direct relationship between mental state & breath translates to certain patterns during "everyday stress:" holding the breath, irregular breathing, short breaths & quick, short exhalations.
Perhaps the most profound shift we could make in ourselves & the world is to – upon noticing that we feel agitated, anxious, depressed or angry – notice our breath. It's quite likely we'll find the unconscious stress breathing patterns. We can then begin to bring ease to the breath, taking a gentle breath in and a relaxing breath out without hesitation or strain.
When we engage in such practices, we're working with the monumental fact that breath is the link between our body & mind. Thoughts & emotions affect the breath. And the breath affects thoughts & emotions.

Natural Breathing
Following are some options for helping students to release patterns that inhibit a full, natural breath.

Breathe through the nose.  
Unless one has a respiratory illness, deviated septum or other reason, the yogic practice is to breathe through the nose rather than mouth.
Sinuses and nostrils filter, warm and moisturize air going into the lungs.
Nose breathing creates more resistance than mouth breathing, providing the lungs with an appropriate time for oxygen extraction and enabling a balanced oxygen-carbon dioxide exchange.
There are times, however, when breathing through the mouth is beneficial. Breath moves more quickly to the lungs and with greater ease and quantity through the mouth.
Allow the belly to move with the breath.
Upon inhale, the diaphragm moves down, pressing on organs & the abdomen. The belly naturally rises.
Holding any muscles in chronic contraction – including abdominal muscles – weakens them. To function properly, muscles must relax between contractions.
While it's true that a strong abdomen supports the back, chronically contracted abdominal muscles will increase tension and stiffness & ultimately weaken. (An example for students is how a bicep curl involves contraction & release. If we hold a dumbbell in an endless curl, the bicep will weaken.)
This is a very important teaching for students who – for a variety of cultural & personal reasons – have tightness or tension in their belly & therefore are not breathing fully. With new students & those under stress, this teaching is fundamental & we might even say critical. Still, this should not be confused with another practice that you may focus on with students who have the ability to integrate both teachings: the practice of keeping a slight tone in the belly between the navel & pubic bone. This is a practice that has many benefits but we won't go into that here since the focus in this section is on the natural breath. We simply want to point out that if you are engaged in that practice, it need not contradict the practice of allowing the belly to move. This is because with awareness we keep the low belly tone while still allowing the upper belly to expand, thereby still allowing the diaphragm to move through it's full range.
Allow the ribs and entire torso to move with the breath.

"The inhalation starts with the gentle swelling forward of the abdomen and then moves upward to expand the rib cage fully. The breastbone rises and swells forward as the shoulder blades slide down your back. These actions increase the distance between the top of the thighs and the bottom of the ribs - the area of the your waist - and it is this increased space that gives the diaphragm freedom to move. As you exhale, allow the ribs to relax and come back to center without losing the spinal length you achieved with the inhalation, and then gently pull the abdomen inward. Breathe like this in all the poses throughout the practice." – Erich Schiffman, Yoga: The Spirit and Practice of Moving into Stillness © 1996 pg 56

Allow the height, width & depth of the whole torso to expand & contract with the breath. Envision not just the front ribs but the whole 360° rib cage. While beginners are unlikely to expand the rib cage fully, asana practice (along with awareness & breathing practices) can help to develop this mobility.

Feel the breath in the low belly, upper chest & back body.
Keep the throat soft.
Allow the breath to be effortless.
Rather than pulling the breath inside with outer muscles of the body, let yourself be breathed.
Let the breath expand in all directions, radiating outward.
Let the breath feel calm & smooth.
Know when to let deep breathing go.
For many of our students, teaching them to maintain a long, full, deep breath is critical to enable them to shift restricted breathing patterns & release the effects of stress.
For those students (and teachers) who enter into deep states of concentration, natural pauses in the breath and a softer breath may occur naturally. In these cases, it is not necessary to deepen the breath. 

Mukunda Stiles explains it this way:
"Provided your concentration is genuine, do not encourage yourself to breathe deeply during yoga asana practice. Only if you are exhibiting holding your breath should you be encouraged to breathe fully. Watching carefully for this distinction can permit you to move into a deeper connection with the process of Classical Yoga. It is often a missing link, a hidden secret, that practitioners disregard in learning how to transition from physical Hatha Yoga to mindful Raja Yoga practice." – Mukunda Stiles, Structural Yoga Therapy: Adapting to the Individual © 2000, pg 58