The torso should not leaned or be arched, especially in the lower back. Begin by dropping the hips while keeping your torso as vertical as possible. Knees should flare out, not forward, and in one movement drive hard through the heels and drive the weight overhead. Keep the weight distributed through the feet and avoid either rocking backwards or forwards off of the toes or heels. At the top, keep the thumb as far back as possible.
To return to the starting position, let the weight drop and bend the knees to soften the drop.