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. While the weight is traveling upwards make sure that you drive strongly enough to fully open the hips, knees and ankles before quickly dropping under the weight while extending the arms. 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.