Lonin

my opinion is that the suwariwaza part allows for focussed feeling of uke's forces and direction without the distraction of movement. Upper body relaxation with lower body rootedness is the core of "internal" martial arts. kokyu Ho movements of arm extension promotes grounding. Wrist rotations promotes the ability to reduce ukes power without losing connection and attachment. Finally movement of nage's core either linearly or by rotation brings about kuzhushi. Hope this will help.