In NLP there are what are called 'Digital' and 'Analog' distinctions. A digital distinction would be like a light switch it's either on or off. An analog distinction would be like a light switch that allows the use to vary the amount of light from dim to bright. In Hypnosis they would refer to this analog distinction as 'fractionation'.
This is when a trance is induce and the hypnotist assists the client to go further into trance and then brings them out at varying degrees (seeing the analog distinction?). This allows the hypnotist to train the client to be more responsive during their session.
President Obama unconsciously does this same thing. When he's repeating a phrase multiple times notice the volume and intensity. At first it starts out and a fairly calm tone. Then the next time that he repeats himself he turns up the intensity and does so with each successive utterance. What he's doing unconsciously is training people to respond to his vocal qualities.
He does this really well because after he's take the audience into more of a fervor and then brings them back down with a particular. He has now essentially trained everyone he is speaking to to response to the vocal qualities of his voice. Now later on in his speech he can make a point and in order to elicit a greater response from the audience all he has to do his turn hype his enthusiasm in his voice much like a person would turn a light from dim to bright.