Read Aadil Palkhivala's reply:
I assume that you mean lying on the belly and then raising one arm, as in Eka Hasta Salabhasana (One-Armed-Locust Pose). Inhalation does become a little difficult in these situations, as you have noticed. This is because your abdomen is resting on the floor with your weight on it, and hence it cannot expand as you inhale. Exhalation, however, should be no problem—in fact, the exhalations generally come out faster than you would like.
As you learn yogic breathing, you should be able to resolve this problem; then you can teach the solution to your student. Yogic breathing means breathing in the chest cavity, not in the abdomen. Practice the pose using the sides and frontal ribcage to inhale, thereby eliminating the need for the belly to expand during the inhalation. This will give you more breathing capacity and control.
Be aware that it is not possible for you, as a teacher, to fully convey knowledge that you have not personally experienced. But while you are learning this technique, you can meanwhile tell your student to take shorter breaths, and advise her to breathe more in her chest than in her belly.
Recognized as one of the world's top yoga teachers, Aadil Palkhivala began studying yoga at the age of seven with B.K.S. Iyengar and was introduced to Sri Aurobindo's yoga three years later. He received the Advanced Yoga Teacher's Certificate at the age of 22 and is the founder-director of internationally-renowned Yoga Centers™ in Bellevue, Washington. Aadil is the director of the College of Purna Yoga, a 1,700 hour Washington-state licensed and certified teacher training program. He is also a federally certified naturopath, a certified <a href="/health/ayurveda">Ayurvedic health science practitioner, a clinical hypnotherapist, a certified shiatsu and Swedish bodywork therapist, a lawyer, and an internationally sponsored public speaker on the mind-body-energy connection.