Wuji, I will try to relate to you what Yang Zhenduo teaches about breathing. Generally speaking, breathing in taiji should be natural, not forced. Don't hold your breath! Sometimes beginners are concentrating so much on getting all the details right that they hold their breath. Once I saw a student faint like this. Breathe through your nose. The mouth is closed but not sealed tightly and the tongue gently touches the roof of the mouth. There are many approaches to breathing in taiji and other martial arts, but in traditional Yang style the emphasis is on naturalness and spontaneity, so that we are not trying to consciously control the breathing. It is true that some movements are not conducive to breathing in and others not conducive to breathing out. For example the part of Needle at Sea bottom where you reach down and squat down to some extent. If you try this you can see it is hard to draw a breath because the position compresses the abdomen. However, it is not necessary to worry too much about when it is correct to inhale or exhale, because as you practice a natural rhythm will result without any conscious effort. Sometimes in the beginning of a student's practice, it may happen that the student's breathing will get stuck, the rhythm is destroyed and he will feel very distressed and uncomfortable. If that happens stop for little while and allow the breathing to become regular again and try to resume practice. In a short time problems like this, if they occur at all, will go away by themselves, especially if you can follow the ten essentials, and a good, strong, effortless rhythm will be established.