Historian James Reston writes in the LAT (on this forty-first anniversary of the JFK assassination) that the steel back brace Kennedy was wearing when he was shot may have actually prevented him from getting out of the way of the eventually-fatal second bullet:

But because of the corset, Kennedy's body did not act as a normal body would when the bullet passed through his throat. Held by his back brace, Kennedy remained upright, according to the Warren Commission, for five more seconds. This provided Oswald the opportunity to reload and shoot again at an almost stationary target.

Without the corset, the force of the first bullet, traveling at a speed of 2,000 feet a second, would surely have driven the president's body forward, making him writhe in pain like Connally, and probably down in the seat of his limousine, beyond the view of Oswald's cross hairs for a second or third shot.