Nothing magical about it.  It's what you might call a "piano celeste."
Temperament strings are all tuned to concert pitch, but one unison
string is tuned slightly flat, just enough to produce a slight beat.
You can also tune the other unison string slightly sharp but that might
be a bit over the top, especially since you have only two strings to
work with below the tenor octave, and eventually just one.

Many [post-WW2] ragtime piano artists recorded on pianos tuned like
that; one that comes to mind is Knuckles O'Toole (Dick Hyman's ragtime
personna), whom you can listen to here:

  http://www.youtube.com/watch?v=zBdHAiXZK4c 

The guy who taught me tuning in the early 1960s (who was already
ancient at the time) told me of having once tuned a piano for a
Paderewski concert.  It was a grand piano, and the soft pedal worked
by shifting the entire action over so that the hammers would miss one
unison string.  He adjusted the thing so that the action's "normal"
position was off in the opposite direction so as to miss the other
unison sting, which he then tuned slightly flat.  Depressing the soft
pedal would then engage the off-tuned unison strings, providing the
celeste effect.  The artist was _Paderewski,_ of all people!

Bill Zabel