ESPN did a great one hour program about this fight. I remember, watching the fight on TV, when this happened I was surprised, but the KO was so extraordinary that didn't matter.
In the documentary, George tells how he took punishment for ten rounds waiting to find Moorer in the right position. When he found Michael in the right place, he thought he would KO him out with the first punch, but hit him on the upper-nose/forehead, and the second punch landed correctly. Moorer said that basically he was KOed with the first punch and after the second punch "my body was dead".
Seemingly it was a sequence of two weak punches, but the amount of energy they carried could KO a horse. You can see that, after connecting the second punch, George's expression shows he knows Moorer was done.
Teddy Atlas was Moorer's coach and he was in his corner that night, and he tells how he saw from the beginning what George was doing and tried to convince Moorer not to fall into the trap.
That was an incredible KO.