The drill, performed incorrectly, can encourage initiating the swing from a pre-flattened barrel. Take note of the demo and observe that the barrel is initially turned from a vertical oriented barrel and not a pre-flattened barrel.
The drill is sometimes referred to as a "fast hands" or a "top hand activation" drill.
No. Top hand stays on top but instead of the thumb being on top as in normal grip-the hand is flipped to thumb down - so the palm will be oriented away toward catcher