I know I can understand Proper English, but I'm out of pressure here.
Thanks for answering so far, but,
"lets you know the pipe for that note has been activated and sounds also".
Is this something someone other than the keyboardist is doing,
something to do with the stops,
or is there a link so that another note, or key, is making a sound,
that the keyboardist didn't do?
I can imagine that if you were pre-programmed for the same song,
but organs aren't built just for one song.
When I saw Deep Purple in 1970,
Jon Lord would stick a knife into the keyboard to depress a note.
Unfortunately, for the big Hammond B3, it got so depressed,
Jon would stand back and throw it so it stuck into the side.
If I remember, it looks like everyone is just sitting around watching and waiting,
while the organist kept playing.
It looks like a big, clear plastic sheet music holder in front and above her,
but it looks empty.