Here is how they explain it;
This new feature extends the dynamic range of highlights by about one stop and improves gradation within highlight areas. By expanding the range from the correct exposure level (18% gray) to the maximum allowable highlight level, the gradation from the grays to the highlights becomes smoother and loss in highlight detail is minimized.
So does it work? Yeah, and i think i'll be using it for all my weddings;
Here is an image with Hilight Tone Priority turned on;
And the same shot with the same settings with it off