--compare-dest option

[mertz.denis at free.fr]

I'm translating rsync.1, and i'm having some problems with the
--compare-dest option description, actually i dont understand the part
which describes the usefulness of this option, it says :

This is  useful  for doing  transfers to a new destination while leaving 
existing files intact,  and  then  doing  a  flash-cutover  when  all
files  have  been  successfully transferred  (for  example  by  moving
directories around  and  removing  the  old directory, although this
requires also doing the transfer  with -I  to avoid  skipping  files
that haven´t changed). 

If someone could explain me this (especially the flash-cutover thing) ,
maybe with a little example, it would greatly help me for the translation. 

Thanks in advance

Denis