What is the purpose of partial (<100%) backlash correction?
The developers explained the use of the M425 code themselves.

We devised a routine for measuring Z backlash automatically during G29
and found that software backlash compensation does wonders for the
first layer. However, this comes at the expense of artifacts on the
rest of the print. In particular, any rapid motions of the motor to
try to take up any backlash will inevitably create a small pause and
vibration, leading to a seam in the print. We devised a smoothing
algorithm that allowed backlash correction to be gradually applied
over a distance, this eliminated any harsh transitions, which again
lead to a huge improvement. Alas, we learned that the feature was very
sensitive to the variances in the printer build, working amazingly
well in some cases, but leading to a degraded quality other printers.
This perhaps is a feature that could be used by someone who wished to
hand tune the algorithm, but not something we could use in a mass
produced printer.
...
&quot;M425 F&quot; sets a value from 0 to 1 which is multiplied by the backlash
distance. This command is meant for use by the slicer, and allows it
to &quot;fade&quot; away the backlash compensation gradually over several
layers, or to turn it off completely after the first layer (with a
&quot;M425 F0&quot;).