Many lifters make the mistake of placing the barbell too deep into the palm during pressing movements like the overhead press. What happens is the bar nests too far back in the hand, often causing the wrist to "break" so that the rep is performed with the bar unsupported by the rest of the arm.
With a conventional grip (thumb wrapped around the bar) it's difficult to correct this positioning and place the bar where it should be, especially as the weight increases. A false grip (where you place your thumbs on the same side of the bar as your fingers) allows you to clasp the bar and put the weight over the wrist and forearm from the very beginning.
False vs. Conventional Grip
The false grip is best reserved for intermediate or advanced lifters as they're strong enough and experienced enough to not drop the bar.