What Chopin meant here was that the pleyrl action was responsive to his gravity-tap . Other “lesser” keybeds/actions didnt give any bounce-feedback (making chopin touch ineffective). He was left only with striking the key, getting whatever “standard” tone was built into the mechanism.
Now given the pleryl action he still had to find his own “form” – namely gravity-tapping. When out of form he played with fingers (vs gravity-taps)
The comment about strength refers to his muscular torsi strength needed for belly breathing. This suspends his coughing (since it minimizes air flow induced by classical breathing) and of course the now primed torso sets up a fluid whole arm response …able to sense and react to gravity tapping.