Turns the device's keyboard on or off locally. If off, the keyboard is disconnected from the device's internal sound generation circuitry. So when the musician presses keys, the device doesn't trigger any of its internal sounds. But, the keyboard still generates Note On, Note Off, Aftertouch, and Channel Pressure messages. In this way, a musician can eliminate a situation where MIDI messages get looped back (over MIDI cables) to the device that created those messages. Furthermore, if a device is only going to be played remotely via MIDI, then the keyboard may be turned off in order to allow the device to concentrate more on dealing with MIDI messages rather than scanning the keyboard for depressed notes and varying pressure.
0 (to 63) is off. 127 (to 64) is on.