http://scrounge.org/speak/burwen/

http://www.audiokarma.org/forums/showthread.php?t=80832

http://www.dplay.com/tutorial/burwen/DNFdesc.html

http://www.dplay.com/tutorial/burwen/DNFsche.GIF

basically it is a variation of a noise gate. the consensus is that though they were great at the time, modern devices + technology have surpassed and outperform them by a large margin.