Mr Schauffler continues:
'The mass (or inertia) of the hand or arm, supported over the keys by its lifting muscles, is of help in taking up the reaction but some added downward muscular exertion, also, will always be necessary if the fulcrum is held still. The fixation for taking up the reaction of the stroke should always be as slight as possible. It should be released between strokes, if there is time, because it is fatiguing and because the joint may be needed to make a free, rapid stroke itself, as when single notes follow chords.'
As for starting the fulcrum at the torso - your wrist will go up instead of the key, then your elbow will move, then your shoulder will go up, then, maybe your torso will stay still!