The singer says that she cannot afford to feel the emotion contained within the song while she is singing it. That her professional presentation of the music, the craft, must come first otherwise she will not fully be the medium, the carrier of the music. The audience feels the impact of the music because of her intermediary skill.
I reach for a parallel from photography, one that until now I have struggled to express to those who cannot understand that an emotional reaction to a subject, - to a child, or a sunset for example-, is not all that is necessary to make a good photograph.
Like the singer, the photographer has a responsibility to communicate as clearly as possible. At the point of communication the audience's reaction is the important part, not the emotions of the maker. How the image is composed is the equivalent to the musical skills of the singer. 'Oh, what a beautiful child, such a splendid sunset', can only be the very first reaction before the maker gets down to the technical aspects of his presentation. The photographers who feel only as they take and process their image are failing in their craft. They must know what ingredients will best express what they wish to communicate. Satisfaction must come from one's skill, not from an emotional pushing forward of a poorly thought through image. Our expression must be channelled through technical ability and knowledge.
Similarly, a writer has the difficult task of taking a raw subject, all bumps and flaws, and presenting it in a comprehensible way to his audience. A musical composer must find a form within which to present his ideas. No one can really afford to get stuck at that first reactive stage, everything must be passed through a refining process of creation. Creativity in the arts is not simply personal expression.