Fluid Pitch is generally a scale locked pitch bend system which means that one can use the pitch bend wheel at any any notes without going out of the chosen scale. This is a crucial feature for keyboardists since it allows them to play melodies with beautiful and artistic bends without having to think about the Pitch Bend Range.
The traditional pitch bend have a constant pitch bend range, usually 2 semitones. Except in a wholetone scale, any other scale such as major, minor, etc would have notes which are not exactly 2 semitones apart. When such notes are played and pitch bend is applied, the current note bends to 2 semitones in the corresponding direction to an out-of-scale note.
To imagine something like this happening for a keyboardist during a performance is not intended and will sound unpleasant to the listeners.
So the pitch bend wheel requires so much practise to get right at all the notes or worst case it never gets used at all.
This is where Fluid Pitch comes into play. By changing the pitch bend range depending on the scale chosen and the note played, Fluid Pitch will bring the destination notes to the chosen scale when the pitch wheel is applied. So that keyboardists never need to think twice before using the pitch bend wheel while performing a complex melody which require bendings.