What does process entail?
Do you mean clean the data like a regular ETL-tool?
Have a look at this article : http://www.tableau.com/about/blog/2014/7/understanding-tableau-data-extracts-part1.
It might help you to clarify your doubt. If it doesn't help, kindly elaborate the term "process" in your question.
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If you connect Tableau to a Raw Data in excel that has chances of discrepancy , Tableau has in built "Tableau Data Interpreter" that can suggest that "Data Does not look Right" ( see screen below ) . So this is the way in which tableau is able to process raw data before visualizing the charts.