Yokomenuchi starts as a sword cut from above the head, but goes in a curve to cut at an angle, instead of straight.
That's a good explanation of the correct technical execution of the Yokomenuchi attack, but what i was referring to was the entry. If we are talking about a swing and not a hook punch the entry, sankaku tai, that serves to stop a Yokomenuchi can stop a swing punch because with a swing punch the attacker has to open his guard more in order to gather more momentum in the punch which means opening in the shoulder and elbow. It's a double edged sword, the more you open the more force you will have but also the greater chance is that your opponent can counter you. Mike Tyson used that punch many times in his matches, everybody would call them hooks because they didn't know any better, he would also get countered many times but that didn't do him any harm and his opponents where usually knocked out because of the massive force that he would gather and off course because he was a strong ferocious puncher.