Narrowing a region of text will copy the region into a scratch buffer in which
it can be freely manipulated. After your changes are done just write the buffer
with :w, :update, :x, or whatever is your favorite way to write a
buffer. Upon writing the buffer the changes will be made to the original
buffer as well. If you have made changes that you do not want to write back
the original buffer just close the buffer with :q!.

Narrow will mark the original buffer as read only and disable modifications.
Multiple narrowed region can be taken from the same buffer. This is useful in
the case of taking the diff of two different regions in the same file. Once
all narrowed regions are closed then the original buffer will modifiable again
(assuming it started out that way).