In MAP he uses slow deep breathing, which is done the 'regular' way, just slowly and deeply. In other modalities, yoga breathing (and in some other of Robert's training) is taught. Also known as 'belly breathing', this teaches to use the lower abdominals to create 'more space' for the air (and to direct energy into the hara) while inhaling, and reversing it on the exhale.
Neither is 'more correct' than the other, they're just modalities of breathing.
You can use belly breathing if it suits you.