arms front, stay without moving back, the thoracic spine and chest pull back, this force opens the shoulder.
step one (04)
some corrections (05)
use the jin to pull the joint, see the hand position as well (06)
step two some corrections with demonstration (07)
HeJingHan BaguaQuan, Baguazhang Teaching Taipei.
See more videos at Youtube playlists
some materials could be added from HeJingHan book:
Bagua Quan Foundation Training