Gentlemen,  I do not believe that the depth of the carbon rod had much
to do with anything.  If I recall my piano playing days, the key always
went down all the way.

What the carbon rod did was (1) establish which key was being struck,
and (2) how fast it went down, the strike speed, hence the pressure.
This changed the length and width of the ink line, the distance the
line widened, which they could interpret as loudness.

Jim Crank