Here's how Yamaha describes it:
"To achieve its superior recording and playback performance, the Disklavier Pro generates extra MIDI data to accommodate precision parameters. All essential data such as hammer speed, key release speed and pedaling is recorded and reproduced within the standard MIDI format, enabling full compatibility with other MIDI devices. For recording piano performances with even more subtlety and detail, you can select the expanded Yamaha XP MIDI format. This utilizes general-purpose controller numbers in the MIDI specification to record additional performance data including key speed and stroke depth, for recordings of enhanced sensitivity and precision. Multi-mode flexibility allows the Disklavier Pro to interact smoothly with existing MIDI configurations and reproduce data generated in either XP or enhanced mode, with automatic selection of the appropriate playback format."
Key speed? Stroke depth? Are these parameters that can be useful to a piano sample (as opposed to a disklavier)???
Just wondering. I'm not sure what they are, actually.
Interesting. I notice that this "XP Midi" is only for the Pro Disklavier midi pianos.
They are saying that when recording a performance, the piano's software uses some general (normally unused) midi controllers (I assume they mean CCs) to store some "extra" data.
That extra data can be used on playback on that piano to generate a more realistic sound.
When talking about "pure" digital pianos, companies like Yamaha use software algorithms they have designed to achieve better playback. They can do this because the software is only for piano. As an example, a lot of people who purchased Yamaha Motif keyboards wonder why some of its piano sounds don't sound as good as the ones in the Clavinova models, even though they are from the same sample source.