This article is based on an article written by Simon Williams
Essentially the vocal harmoniser shifts the pitch of your voice, as fed into the keyboard from a microphone or external line input. However as well as pitch shifting, the actual timbre or character of the sound is also changed using a digital signal processor. This enables the shifted signal to sound more natural, as a woman or man rather than a speeded up micky mouse type sound. The pitch of your voice is shifted according to the chord played in the accomp left hand section of the keyboard notes in a song channel (harmony line) or a fixed interval (not related to notes played)
Another option will ensure that your voice is always on key, it does this by shifting the pitch of your voice to the nearest semitone.

Each preset harmony will change these parameters in different ways. A harmony is produced by first of all producing pitch shifted versions of your voice and then mixing them with the original sound from the microphone. The amount by which each voice is shifted (there are upto 3 voices I believe) will be according to the harmony type. The root pitch from the harmony is derived from the chord played in the accomp. section. For example if you play C major, then a typical harmony preset may create extra pitch shifted copies of your voice at E and G notes.

  Different harmonies produce different intervals, similar to the harmony effect on the keyboard . In addition, the extra voices produced may be male or female. Some harmony presets produce unison effects (2 or more voices singing at the same pitch), others produce female only, male only, combinations or simply correct the pitch of your original voice to the nearest semitone.

Towards the end of the list are other options. Sing XG will play a note on the keyboard of the selected instrument according to the pitch of your voice as you sing into the mic! Although this doesnt work so well with vocals, its quite interesting if you feed a guitar or other instrument into the keyboard. Speedy Mouse will shift the pitch of your voice upwards but without the female signal processing, so it sounds like a tape at fast speed. The Karaoke presets perform pitch correction to your voice without harmony. Sing bass will add a sub octave bass voice to whatever you sing, tracking the pitch. There's lots of variations on each theme, but they are all combinations of pitch shifting at different intervals, pitch correction to the nearest semitone, vocal timbre change (male or female) or triggering the keyboard from the pitch of a signal input.