Related: How do I quickly open or close a jammed zipper?:
In an emergency situation, what would be the fastest way to open a zipper, provided that the use of the pull tab is disabled, and access to the top stop and box pin are inaccessible?
(source: zippersource.com)
Neglecting issues of damage to what lies behind the zipper, is there a specific method by which to easily separate the teeth with an instrument such as a knife from the middle? Where easily is defined as factor of minimal applied force and accessibility of said tool.