Thiem uses a platform stance, that is he does not step forward with his back foot to serve. However, he starts with his feet very close together exploding up to the ball as if using a pin point stance. We see a perfect power position, knees bent, body sideways, left arm pointing at the ball and racquet head pointing up with the arm in a throwing position.
From this position he pushes up and starts dropping his racquet behind his back. The racquet head reaches its lowest point. Throughout the swing he remains sideways, the back hip rotating over the front one, which allows him to land on his front foot with the back foot in the air (Horse Kick).