Right as you lowered your arm to hit Lars' gut, you realize that now your arm is over-extended. You are unable to defend your arm as you go downward with it, and Lars knew this as it happened.

With a lightning blow he hit your arm with his sword, forcing you to lose that arm. You drop your shield and put your sword into your arm. Since you are right handed, and your sword is now in your left hand, this is going to be a difficult battle.

Bors calls "lay on!" once again as you finish changing the arm your sword is in and put your now dead right arm behind your back. As the next part of the fight begins, Lars swings his sword from above, which appears to be a shot at your now undefended chest.

In the split second of the beginning of this attack, you notice that Lars' shield has dropped to his waist. Obviously, this guy is all speed and no coordination! Lucky you! You can either try to hit his arm which is unprepared for defense, or you could try to maneuver around that sword of his and cut his chest.