Phoenix Reborn is much like the Body Shedding technique used by Orochimaru ; the main difference between the two is the manner of how the new body materializes. The users body is engulfed in flames, making one believe that the user had died as ashes are left behind. Ashes being already burned aren't effected by techniques such as Amaterasu making a great shield for the new body created within the ashes and allowing escape.