I'd say it's all inside you... although any force that your opponent provides can and will be used against him.
Kuzushi on contact requires a combination of "shape" (i.e. you are forming your frame and structure in such a way that it is arc-like so that force can be directed by you and is not dumped into any of the joints, which would cause structural failure or require you to step, bend or otherwise move away from the force to maintain or restore your balance), the manipulation of your "yin" muscles to absorb and neutralize incoming force and of your "yang" muscles to propel force back into your opponent. The simultaneous absorption and propelling of force temporarily "freezes" the opponent. If you do this process rapidly/explosively, it creates that "shocky" effect. Even more so if you add the diaphragm-dropping fun fa-jing.