The left shoulder dips forward by bending at the waist and flexing the left knee. The power comes from the combined drive of the left leg, hips and shoulders with the left arm adding to the momentum with a whip-like movement.
The palm of that love faces inwards, thumb on top, as the punch is driven into the opponent's solar plexus.
Protection is provided by a right hand which remains in front of the right shoulder to block any counter.