Normal abdominal breathing (NAB) is a mixture of Yin and Yang elements. When you inhale, you draw the breath in (Yin), but you gently expand the abdomen (Yang) and gently push down on the Perineium aka Hui Yin or Pelvic Floor (Yang). When you exhale, it’s just the opposite but still a mixture of Yin and Yang.
Reverse Abdominal Breathing (RAB) does not involve mixed Yin and Yang elements. When you inhale, you draw the breath in (Yin), you gently pull in on the abdomen (Yin) and you gently lift up the perineum (Yin). When you exhale, it’s just the opposite and consistently Yang.
RAB works more efficiently to move vital energy (Qi) because it, unlike NAB, does not mix Yin and Yang elements. Therefore Taijiquan and Qigong practitioners will want to use RAB whenever they have a desire to move their Qi strongly.
BTW the movement of the perineum, which occurs in both RAB and NAB, is an important key to moving the Qi, also adds a massaging effect for the organs in the abdominal cavity. With NAB, the diaphragm employes a greater range of motion up and down than it does with chest breathing. In RAB, the diaphragm moves down as you inhale while at the same time the perineum moves up, which increase the massage effect and helps promote the circulation of blood and qi in the abdominal organs.