Disklavier

If you have a small silent control box on the left  side of the piano 
opposite the Mark III  it lets you either turn off  the piano and play with the 
accompaniment or adjust the volume of the  accompaniment when the piano is also 
playing a PianoSoft Plus disk. It  alos has a headphone jack so you can listen 
without anyone else hearing  the piano.
No all Mark III pianos have this feature. I think only  the Mark III Series
Full-Function Pianos have it.
_http://www.yamaha.com/yamahavgn/CDA/ContentDetail/ModelSeriesDetailPF/0,,CNTI
D%253D2731%2526CTID%253D201700%2526LGFL%253DY,00.html_ 
(http://www.yamaha.com/yamahavgn/CDA/ContentDetail/ModelSeriesDetailPF/0,,CNTID%3D2731%26CTID%3D20170
0%26LGFL%3DY,00.html) 
 
Refer To the Mark III full function manual A page  18.
_http://www.yamaha.co.jp/manual/english/result.php_ 
(http://www.yamaha.co.jp/manual/english/result.php) 
 
Digital Quiet/Silent mode flexibility
Yamaha Silent System™ technology lets you  instantly cancel the acoustic 
piano sound and switch to digital piano tone  generation. This means that you can 
play or listen in Quiet Mode  through the built-in speakers, or in Silent mode 
via the privacy of  headphones, letting you enjoy the Disklavier at any time 
by selecting a  sound level suitable to the time of day and the activities of 
those around  you.

By eliminating the sound of the acoustic piano, this feature  also lets you 
play internal orchestral voices independently – ideal for  multi-track 
recording or use as a MIDI controller. And in digital mode,  you can easily tune the 
piano to match the pitch of other instruments or  recordings.

Stereo sampled from the Yamaha concert grand, the  digital piano tones are 
virtually indistinguishable from the original,  while a special dual-mode action 
mechanism maintains traditional keyboard  touch and response. Variable reverb 
effects can simulate the acoustics of  various playing environments, and dual 
headphone jacks let you share a  private performance with another, if you  
like.