see this part
The 盤旋陰陽掌 (Pánxuán yīnyáng zhǎng) movement is belly, the force comes from there.
More details about it … more comes later..
HeJingHan BaguaQuan, Baguazhang Teaching Taipei.
See more videos at Youtube playlists
some materials could be added from HeJingHan book:
Bagua Quan Foundation Training