Visual Totals is a Grid Property, which enables any parent member to show the total for the members selected in the report.
For example:
Without Visual Totals enabled:

The value for All Geographies is based on all the members in the cube.
With Visual Totals enabled, All Geographies is based on the selected members-only Canada, United Kingdom, and United States.

Restrictions
The order of the selected members is very important for how the totals are calculated. For the members to be totaled, they must appear in the member selector with the highest member before the lower-level members:
The selections below would total the members correctly to give the aggregate of the three countries:

The below example will display the total for All Geographies as it appears underneath its nodes:

Here's another example where All Geographies has France as a child node.

You can't include the Parent or Ancestor members twice. If you try to include All Geographies twice – it won't work and will be displayed with visual totals enabled (even if you try using Unions). This is a restriction with Analysis Services.
Visual totals are calculated before Sorting and Unions
If you want to display the Visual Total for All Geographies, you can do this using the Hierarchize sort with 'Parent after children' selected:


It only works if you have a single member selection and no ranking, filter, or exclusions.
Visual totals work only with unsorted ranking and filter
You can set up to show the top three countries and the totals, but then you can't sort the result to show the total at the bottom.
Note: System uses top four to include All Geograpies and then the top three countries.