The aim of this filter is - guess what - to remove red eyes from an image. Before applying the “Red Eye Removal” you must do a selection (lasso or elliptical) of the boundary of the iris of the eye(s) having a red pupil. After only you can apply the filter on this selection. If you don't make this selection, the filter inform you that : “Manually selecting the eyes may improve the results”.
If you check “Preview” you can see the modifications in real-time in the preview window. And you can choose the good value of threshold compared with what you see, and then validate it.
If you move the cursor of threshold the amount of red color to eliminate will vary.