I vaguely recall reading somewhere (maybe on here!) that if you are fielding close in on the off-side then to jump if the ball is hit at you hard as batsmen generally try and keep it down on the off-side, and to duck if leg-side as batsmen will often not worry so much about keeping it down. Also don't turn your back - all your padding is on your front, box, shin pads, helmet protects front more than back, possibly chest guard - if you turn your back you are exposing more unprotected areas especially your neck.