compare-windows-sync is a variable defined in `compare-w.el'.
Its value is

Function or regexp that is used to synchronize points in two
windows if before calling `compare-windows' points are located
on mismatched positions.

The value of `compare-windows-sync' can be a function. The
function's job is to advance points in both windows to the next
matching text. If the value of `compare-windows-sync' is a
regexp, then points in both windows are advanced to the next
occurrence of this regexp.

The current default value is the general function
`compare-windows-sync-default-function' that is able to
synchronize points by using quadratic algorithm to find the first
matching 32-character string in two windows.

The other useful values of this variable could be such functions
as `forward-word', `forward-sentence', `forward-paragraph', or a
regexp containing some field separator or a newline, depending on
the nature of the difference units separator. The variable can
be made buffer-local.

If the value of this variable is `nil' (option "No sync"), then
no synchronization is performed, and the function `ding' is called
to beep or flash the screen when points are mismatched.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.