The rationale for the exercise is: what do you do if at the beginning of the fight, your right hand is injured and unable to work a gun? Do you accept being killed, or do you use your left hand?
Your right hand might have been disabled before the conflict (for example, you might be walking around with a broken right arm), or it might have been disabled at the beginning of the fight (bad guy cuts you in the arm with knife) or during the fight. It's good to be able to continue the fight one-handed.
It WAS, after all .. HIS foot !