One of the greatest punches in boxing history. There is so much going on in that second. Here is a better gif/video that shows the shot from more angles:
http://gfycat.com/DistantWelcomeAzurevasesponge
To start with, it is interesting that Pac is circling to his left. When a left handed fighter circles to his left against a right handed opponent he is essentially challenging that fighter's right hand. This is a counter-intuitive move but it is one of Pac's signatures and he utilized it brilliantly against Bradley in their second fight. He'll normally feint and then jab with his right hand (or throw two jabs) and move subtlety to his left so he can land his strait left, even if his opponent throws a counter right. The idea is to establish an angle where he has enough range to throw his left but his opponent does not have the range to throw his right. He uses the feints and/or jabs to get in the correct position relative to his opponent. Here're where Marquez's mastery comes in.
As soon as he sees Pac's feint he anticipates the 1-2 (jab, straight-left) that will follow and he steps to his own left the second before Pac steps to his left and in the same motion he puts his entire body into a chopping overhand right. This is a counter that was only able to connect because Marquez had so effectively learned Pac's range and timing over the course of the 40 rounds they had spent together in the ring. In order to land the way that it did, he needed to initiate his counter
before Pac even threw the jab. Also, when the punch lands, Marquez's fist is even with his own head! Just try to throw a punch like that. He loops it over the top at such and angle that it could only have landed if Pac were exactly in that spot. And, again, he initiated this movement/punch at the moment of Pac's feight
before Pac had even threw the first punch of his 2 punch combo. Pac's was going for feint-jab-left and Marquez was able to feint and then throw and land that chopping right because he had Pac's movement and timing down exactly.
One more note: if you look closely at their feet, you will see that Pac actually trips over Marquez's left foot. When Marquez takes that movement to his left, he moves strait into the path Pac's attempted movement. Because of this tangle, Pac is actually falling forward as the exact moment that Marquez's punch connects. This is the reason that it lands with such an impact and Pacquiao was effected like to no other time in his career.
Magical stuff.