In a Yin Yang yoga class we work with both qualities because active, dynamic movements (Yang) are alternated with stilling postures (Yin) that are held longer just like in the Yin classes. The Yang in the movements is focused on the muscles and provides a powerful vital body. The Yin postures focus more on the deeper connective tissue around joints and organs and brings you into contact with the silence within yourself. This creates a healthy balance between body and mind.
Pay attention if you are pregnant or have just given birth, always let the teacher know. Since your body is soft it can be harmful to go deep in the postures.