Dakota Mitchell, personal trainer, replies: You win. When your hands are together it causes too much impingement on your wrists. It also forces your elbows out as you bring the weight down creating rotation, which can lead to injury. Finally, this grip doesn't even put your triceps at a good mechanical advantage.
To perform the move properly, use a shoulder-width grip, hold the bar at the bottom of the sternum and keep your elbows pointing towards your feet. When you press up, keep the effort focused on your triceps rather than on your shoulders or pecs. Also, try to press with the meaty part of the palm, which will keep you from rolling through the wrist. You get a tremendous activation of your triceps and it ensures your wrists and elbows are a lot less likely to become injured.