Here’s what Yamaha say
The CLP-645 is the mid-range Clavinova CLP, offering powerful sound and a superb Natural Wood (NWX) keyboard action. It features the sounds of the flagship Yamaha CFX concert grand and the Bösendorfer Imperial that sound pristine through its bi-amplified internal sound system. And the Clavinova Virtual Resonance Modeling emulates the more complex elements of playing these acoustic instruments. Bluetooth audio lets you stream music from your portable smart devices, wirelessly!
The Clavinova keyboards feature an escapement mechanism that reproduces this sensation near the bottom of the key dip. They have been designed in such a way that the click is discernible only on the lightest keystrokes, similar to the keyboard of a grand piano.