When you are displaying more than one buffer in the same window, you can set the scrollbind option in each buffer so they scroll together.

In each buffer that should scroll simultaneously, enter the command:

:set scrollbind

You can enter scb as an abbreviation for scrollbind, and the ! flag causes :set to toggle a boolean option. Therefore, it is convenient to enter the following to toggle scrollbind on or off:

:set scb!

use the :diffthis command to initiate a diff when Vim is already running.

e.g. file is open, open new file in split screen, invoke diff mode in both splits with :diffthis and turn it off with :diffoff.