Compression
Open / Close
Pay attention to the opening and closing of the ribcage
Try doing your form or a section of it without using your arms. Focus instead on the turning and sinking of the torso to understand how this leads the arms and legs.
Do this a few times and then do the form as usual with greater awareness of the role of the torso.
I like this short clip by Scott Meredith, where you can clearly see how the waist leads the sword.
It's a great training exercise that you put into your solo form.