The LUT is basically a more sophisticated version of the FAT which can be programmed with a second gyro angle which initiates after it completes its straight run and just before it starts its ladder pattern.
This is best explained with a diagram, if you follow this link and scroll to the middle of the document (the section concerning torpedos) you'll see the difference between FATs and LUTs explained in detail with pictures.
http://www.uboatarchive.net/KTBNotes...iveEdition.htm
It's from an allied wartime intelligence report so some of the information in the document is speculative (distinctions are made in the text between what is believed to be fact and speculation, makes for interesting reading), although the description of FATs and LUTs is pretty much bang on as far as I know.
edit: And yeah like derrinurban said... it's in the GWX manual too.........