The main reason for this 'head lifting'
is that when the gun is fired, the huge pressure created by the expanding
gases in the cartridge cause the barrel of the gun to lift, known
as 'muzzle flip' and the comb of the gun is forced at extreme speed
into the shooters cheek.
Once this has happened a few times, our self preservation reflex will
initiate a recoil reaction involuntarily moving the face away from
the upwardly moving comb. A movement of just a few millimetres can
make a big difference to our eye / rib alignment.
This reflex is something that no shooter
can override. Our brain will not allow us to repeatedly cause pain
to our bodies so we naturally adapt to this by placing the face on
the comb in a way that gives us little or no discomfort as we fire
Unfortunately, any location of the face to the comb that compensates
for the muzzle flip will also allow for misalignment of the eye /
rib relationship that is critical for accurate shooting.
It is difficult enough to address each target correctly, read it's
line of flight and pull the trigger when the appropriate amount of
forward allowance has been given, without the added problem that the
gun is not pointing exactly where we think its is.