This plugin evens the tone of an image. It useful in photography for correcting weird light, and can hide skin blemishes.
The layers are not merged to allow opacity adjustments after the filter has been run.
Usually for a photograph, you'll want to use either the sharpen layer, or the "Soft Light" layer. Using both can make colors look too saturated, and has a posterizing effect.
For extra sharpness, change the sharpen layer mode to grain merge, but be ready to lessen the opacity, as the effect is quite striking.