The Yamaha PF1000 keyboard features the Natural voices you are asking about. I've copied a few sentences from the Yamaha website where they explain their Natural voices.
George Kaye
Kaye's Music Scene
"For even more realism, the PF1000 features Stereo Sustain samples to recreate the resonance of the strings and soundboard when the damper pedal is pressed, and Key-off samples to simulate the subtle sound that piano keys make when released.
Natural!" voices--pianos, electric pianos, harpsichords and other keyboard instruments--have their own stereo-sampled waveforms, including Dynamic Samples, Sustain Samples and Key-off Samples, for truly expressive performance capability. "Sweet!" voices accurately emulate the natural tone and vibrato of acoustic instruments such as brass and woodwinds. "Cool!" voices realistically reproduce the dynamic textures and subtle nuances of electric instruments. And "Live!" voices are stereo sampled with natural room ambience to ensure maximum realism and expression."