The user steps in with his left foot, and pretends that the main attack is a punch to your opponent's head, while actually simultaneously punching stomach. This attack is forbidden in sport karate.
Yamatsuki is (unlike in the manga and anime) supposed to hit with both fists, and will never be used with the intension of sacrificing the upper punch (as a feint) to hit with the lower punch.
Hitting someone with a well placed yamazuki shocks the entire body, preventing someone from being able to continue the fight.
Targets of the upper and lower fist can vary, depending on the preference of the user, and the position (in front or at the side of the opponent).
Possible targets for the upper fist: the top of the chest (between the collarbones), the front of the chin, a pressure point located between the upper lip and the nose, the nose, the side of the chin, and the temple.
Possible targets for the lower fist are the stomach, the kidneys and the lower ribs.