At first blush this might seem like an illogical idea, since JFK was killed by a wound to the head. But the theory revolves around the fact that JFK's back brace kept him from slumping over when the first, non-fatal shot, hit him. Thus Oswald was still able to target his head for the fatal shot.
How might history have changed then, had JFK not needed that back brace? One could even speculate that he died because of that fateful night in August 1943 when the Japanese destroyer Amagiri struck PT-109 and sliced it in half, though that may have only been exacerbated back problems he already had.