Rockstar finally nailed the targeting system. Holding the left trigger all the way locks on your target and the camera goes into RE4 mode. While locked on it works similar to crackdown. From hear just flicking the right stick will target you specified body part. Heres where its different from crackdown. You cant just flick the right stick up and lock on for head shots. You're required to slightly hold the right stick in the desired location otherwise it zeros back in to the center mass of your target.
God I hope I explained that well. It will make so much more sense once you get you hands on it.
Its pretty funny to shoot a cop in the leg and see him limp to the back of his squad car for cover.