It is the man that turns the lady's head. His body shape(sway) causes the lady's head weight to move from side to side in turns ,lines etc, but the lady's neck has to be flexable enough to have this happen and both man and lady have to be aware of where their own and partners body and head weight is throughout the dance. So first the head weight moves followed by the turn of the head, but untill the man has learnt to sway the lady has to move her own head. Unfortunatly many women continue to turn their own heads at an advace leval and that is why it looks so unnatural or forced. The lady should be able to feel this when dancing with her teacher.