The harmonic trap on the upright soundboard (if present) is a thick
strip of wood glued diagonally on the top right (on the bass side, when
looking at the soundboard from the rear of the piano).  In most cases,
the ribs stop at this point.  On some pianos, the soundboard area that
is above the trap is missing altogether.  On others, there is a top
harmonic trap, and a lower trap located at the bottom treble side of
the soundboard.

According to Art Reblitz's book, "Piano Servicing, Tuning, and Rebuild-
ing", 2nd edition, Vestal Press (see, I told you that book is a must
read!), the harmonic trap helps to suppress undesirable harmonics,
ringing and other extraneous sounds.

I have never experimented with soundboard traps to find out what
difference it would make in the tone of the piano if the trap was
removed from the soundboard, but I did notice that the Farrand player
did not have one.  Hope this helps,

Andy Taylor
Tempola Music Rolls