Do you mean something like you can see on the F-22 on this site?
http://www.edwards.af.mil/gallery/html_pgs/f-221-99.htm
Take a look at the 4th one down.
As far as I can see its used in case the aircraft undergoes an unrecoverable departure from controlled flight. It is a parachute that is used so the pilot can regain control.
Hope this is what you meant.