Can someone explain this to me?

[GrandpaBernard]

That looks like the punch in the 2nd round that Mosley hurt Floyd with, but it doesn't hurt Floyd just shrugs it off this time.

Did Mosley not have the right distance to generate proper leverage for power? Did Floyd just simply see it and brace himself?

[The Surgeon]

I cant see the gif to well on this laptop but he might have rolled with the punch, taking the snap out of it. Or sometimes a fighter after getting tagged early simply warms to the task and seems to just handle the shots better

[Wild Blue Yonda]

A lot of fortune is rolled in with technique & skill when landing the right punch for the right effect. Mosley was just at the perfect range for that shot which buckled Mayweather. I was watching Mayweather-Hatton a few months back, & I noticed the, "check hook," Mayweather landed which effectively ended Hatton's night in the tenth is a punch he lands on an on-rushing Hatton's jaw over & over throughout the fight --- there are some hellacious-sounding ones, too. Hatton never once looks shaken, yet in the tenth, the same sneak-shot Mayweather has been landing all night turns cataclysmic.

Luck plays its part, in addition to all the skill, timing & talent which also gets rolled into the perfect shot.