Digital Light & Color

What is exactly being blurred in BS if Sharpen Luminance Only is ticked on?

Also, I don't understand how Halo Limit interacts with other settings in BS. For example, if I just want to see what BS is going to blur (Sharpen Factor = Shapen Threshold = 0% and Blur Threshold = 100%), the Halo Limit sliders seem to affect blurring step itself - I didn't expect it...

For luminance only, the image is converted to HSV, V is sharpened, and then converted back to RGB using the original HS channels.

The halo limits cap the amount the result image can differ from the original in either direction.