Chris Webb shows a new feature in SSAS Tabular vNext:
What drillthrough does in SSAS Multidimensional, and what the new Detail Rows Expression property in SSAS Tabular v.next does, is allow an end user to see the detail-level data (usually the rows in the fact table) that was aggregated to give the value the user clicked on in the original PivotTable.
Read through for an example as well as how it’s already an improvement over Multidimensional’s dillthrough.