Let's first assume you have two dimensions, Year and Quarter.
When you click on a bar the whole stack will be selected unless the outer dimension (Year) is locked. If it's locked then it's no longer possible to select the whole stack, instead the selection is applied to the inner dimension Quarter.
Another way to select on the inner dimension is from the color legend which shows Q1, Q2, Q3, Q4. When clicking on the quarters in the legend the whole series will be selected and highlighted in each stack.