Basically it is not a plugin. It is a small standalone program in C++ and a wrapper script for seemless integration into GIMP.
It allows to alter an image with 16 bit L*a*b* curves with preview. You call it like a script from gimp. The image is send to that program, you play with the L*a*b* curves and when you save the altered image is send back and everything is cleaned up...
More information here: http://www.mm-log.com/lab-curves-gimp