What does the Highlights slider on the Post-Crop Vignette do?

[JBedfordPhoto]

Maybe this is a dumb question, but I've tried using the slider in a variety of post-cropping scenarios and can never quite see what the tool does. 

Can anyone answer that?

[Rob_Cullen]

Increasing the 'Highlight' slider is 'protecting' the highlights where the vignette is acting, thus keeping up some contrast in the vignette area. It may prevent the vignette looking 'flat' and low contrast.

(If you have no highlights under the vignette- I guess there will be no effect 🙂 )