It is a simple variation of a regular change up, and when thrown and controlled well, it is extremely effective.
It is so effective for me because It looks exactly like my 2 seam fastball, and that is the fastball i use most of the time (bcuz i only lose 1 or 2 mph, and get insane movement).
My personal Vulcan Change Looks like a 2 seasm fastball, and moves like a splitter/screwball (seriously, thats why i throw it). It dives sharply down and in on righties, and lefties might as well not even bother swinging at it.
I cant rly explain it, i would need to show you. BUT if you want to see how it is thrown, check the pitching articles section. There are 2 articles, one on how Gagne throws it, and one on how some old Pittsburgh guys threw it.
Sorry i cant be more help