This is used to determine exactly what has changed in two text files.
You load the two different versions in two different buffers.
Position the two buffers such that the top lines are the same.
Then, you hold down whatever key is used to switch between two buffers.
On a fast machine, this causes the characters which are different to flicker. The characters that are the same will remain steady.
(Thanks to DavidHoffman? of IBM for this tip.) -- RonPerrella