New in version 2 : The biggest change is that the three parameters (highlights, shadows, amount) has been replaced by a more useful one named High Frequency Impact (few explanations in first example).

Here is a brief description of the three equalization methods, and options:
Contrast: Usually the most reliable method. Apply a single curve. Notice that the equalization curve might differ , if you check the Preserve hue option.
Levels: This method can help sometimes to remove color cast. Because it applies a different equalization curve to each channel, it can also lead to the apparition of new unwanted colors. To limit the damages, the option Restrict Color Shifts was added.
Luminosity: Only equalize the luminosity. Three different ways to calculate the lumosity are suggested (use the small slider). The value Perceptual is the most representative of our perception of luminosity.