Furthermore, he suffered from sever back pains for most of his life and at this instant was wearing a stiff brace to support it. With this in place, he wasn't able to slouch as would be expected after the first shot through the neck, but rather held him erect and vulnerable to the lethal shot. This rigid support also possibly helps give the appearance of the 'third shot,' back and to the left.
Cooper, Pauline. The Medical Detectives. New York: David McKay, 1973. ISBN 0-679-50382-X @ Amazon [a] p. 198 [b] p. 209
Dallek, Robert. The medical ordeals of JFK. Atlantic Monthly. 2002 (Dec);290(5):49-61. [a] p. 61 [b] p. 60