During the past 35 years I've come across many uprights in which the
keys do or don't move when a roll is played, and, except in the rare
cases where a piano was so damp nothing moved freely or it had been got
at by an ignoramus, this was entirely by design.

Remember that in a normal upright action at rest there must be a little
play between the jack and the butt so that the jack can pop back in.
This can only happen if the hammers rest on the rest rail and the
whippens can fall back under their own weight.

Similarly, for good repetition, the whippen must return quicker than
the unchecked hammer for the jack to go back in.

In a tall upright the leverage of the whippen with the weight of its
attached prolonge (sticker) is usually sufficient for it to return
smartly, even with front-heavy keys.  But in a short upright with no
prolonges or one with capstans on tall key wires the keys are often
made back heavy; they can be front heavy, but the leverage of the
whippen must be enough to return the key swiftly or the section will
block.

The action designer and finisher will have decided this to a nicety
so that repetition is good but the weight of touch not too great.
Reweighting back-heavy keys to make them drop when a roll is played
invariably spoils repetition in both hand and roll playing, as in
Christine Robinson's Gruenert.

In a player, the whippen's return speed can be compromised because to
an extent it may have to push its striker pneumatic open against any
set in its rubber cloth or air resistance if the valve clearance is
minimal.

Therefore many player makers fitted a key lock to remove unnecessary
extra key weight and inertia and improve soft repetition, especially
when pedalling rolls -- which remained popular in the UK even after
reproducing pianos arrived, hence the pedal-electric Duo-Art.

Many player manufacturers used actions by several different makers even
in the same model, and sometimes gave different key and touch weight to
different marques, but fitted a key lock anyway, as with the Gruenert.

I'm inclined not to believe the notion that the British thought the
keys moving improper: we readily accepted normal under-action grands
in which the keys must move.  However, Welte grands always have a
compulsory automatic key lock that operates when a roll is played.
Since they have over-actions this may be to aid repetition, but their
cunning valve system makes soft repetition exemplary, so perhaps it was
the Germans who were rather more stuffy.

Patrick Handscombe
Wivenhoe, Essex, UK