Connecting to #powerpivot from an external program (such as #Tableau)

Many people requested me how to connect to Power Pivot from an external program, without publishing the workbook on SharePoint or on Analysis Services Tabular. I always said it is not possible (for both technical and licensing reasons), but someone observed that Tableau is able to extract data from a Power Pivot data model connecting directly to the xlsx file. I wanted to investigate how they solved the limitations that exists.