Although a
punch starts out its
life with a somewhat circular path, the real meat and potatoes, the
part that
really matters is linear. 
A correctly executed punch begins
at the waist. With a small
backward circular
motion it is brought to armpit level. The
arm is then extended straight out. At
the end of the punch the elbow should be slightly bent and the arm
level with
the shoulder.
The force of the punch is
dependent on the mass and acceleration.
F=ma
m = mass
a = acceleration
http://1.bp.blogspot.com/_1V7wnZxPqok/RpZjWdrDayI/AAAAAAAAFmg/MT4HHbvCqkc/s400/cartoon+punch.jpg
There is
only a limited amount of
mass in your arm. In order to get the
most force behind a punch the attacker needs to get the mass of their
whole
body into the equation. This is
accomplished with the use of specific stances and body movements.
The body movement is called sine wave. Sine wave is a down-up-down motion that occurs while the attacker is moving forward that creates movement in the whole body and puts more mass behind the punch.
The
attacker can draw on the mass
of the entire earth as well. This is accomplished by pushing with the
back foot
on the ground as they execute the punch.
Thanks to Isaac Newton we know that the force applied by the
attacker on
the Earth creates a force of the same magnitude by the
Earth on
the attacker. This can then be translated through the attacker's
body and added to the force applied to the board.
There is a
physics related reason
for the bent elbow as well. A completely extended arm at the moment of
impact
would mean that the acceleration and momentum had stopped, or at the
very least began declining, before impact. In
order to
maximize the force of the punch, impact should come when the
acceleration was
at its greatest.
The
attacking tool of a punch
should be the first two knuckles of the hand. This
limits the area that force acts over thereby increasing the pressure on
the board.
Striking the target with the proper attacking tool brings the
whole arm
into alignment. This strengthens the arm
and prevents it from buckling.
Taken by A. White. Used with
permission.