Half the mind concentrates on the movement itself, the other half of the mind is empty or open. In the early stage you pay attention to the movement or the qi itself. At the advanced stage you don't have to pay attention to it, all the channels are open and the qi is flowing naturally with concentrating on it. This is because the energy moves easily through the body. No matter what stage, it is important to keep one half of the concentration on the body's movement and one half of the mind open.
Different postures require different coordination of the muscles, resulting in different emphasis.
Qi is communicated through the channels, so there are different results with different movements. Many people ask this. There are same and different principles associated with each form, but one should not try to control the breathing. Keep it natural. When doing fajin, it is natural to exhale.