In the first Jeopardy book, Alex wrote that there's a neon light on the side of the board, that alerts contestants when it's safe to buzz in, and if they buzz in before the light comes on, they'll be locked out. This light can also be seen during the J! scene in "White Men Can't Jump*."
Does J! still use this light, and for Matt or Ken, if not, then is there still a signal used to inform contestants when it's safe to buzz in?
*It's a movie title; I have no concrete evidence that white males are, in fact, incapable of jumping. ;-)