Here is a great article by Tony Blauer the founder of the PDR system. It gives more insight to the S.P.E.A.R. system.

Another good article about what makes the S.P.E.A.R. system so effective and easy to learn.  The ability to take a natural reflex or flinch and turn it into a tactic that is both natural and effective can not be understated.  Don’t let the simplicity of a move undermine the effectiveness of its outcome.  You see, fancy and complex motor skills can take years to develop and then in a real attack, when adrenaline is pumping full force, the last thing on your mind will be how to execute a fine or complex motor skill.  Gross motor actions are what is needed to to have the best chance of survival.  I love watching multiple black belts or life long experts in certain arts videoed taped  in a real fight ( actually lets define “real”, they are usually matches in the ring) , they are just swinging hay-makers and punches, etc..  all fine motor techniques go out the window fast.  Are there some guys who can pull them off?  No doubt. Would you want to put your life on it? No way.  Gross motor skills and bigger targets in real situations have the best chance for success in my opinion. Read the article for greater insights.