This filter curls a corner of the current layer or selection into a kind of cornet showing the underlying layer in the cleared area. A new “Curl Layer” and a new Alpha channel are created. The part of the initial layer corresponding to this cleared area is also transparent.
You have there four radio buttons to select the corner you want raise. The Preview is redundant and doesn't respond to other options.
Horizontal and Vertical refer to the border you want raise.
This is the shadow inside the cornet.
This option refers to the outer face of the cornet.
Refers to the visibility of the layer part underlying the cornet. It may be set also in the Layer Dialog.