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Saïx used the moon to enrage his anger against those who dare step within his restraining order with a heart and uses his power of holding Alt and clicking 1 3 9 to bestow the power of the double i! And memories...I think thats how he does it...
Saix is a person obsessed with rage if you listen to his dialouge throughout the game. He simply pretended to have emotions, in this case rage, to improve his combat abilities. An emotionless person like himself only analyzes what emotions he believes are superior, and in his case it was rage.
Xemnas also believed anger and hate were supreme, although he's the only character to never show emotion in dialouge nor battle. Saix only showed emotion during battle, which is probably why he was 2nd in command.