This works because, the INDEX function returns a reference to the first cell, not the actual value.
The result is a reference that expands.
The SUM function sums the values in the range at each row, creating a running total.
Simple expanding range
Why not use a simpleexpanding rangelike this?
For some reason, this kind of mixed reference becomes corrupted in an Excel Table as rows are added.
Using INDEX with a structured reference solves the problem.
it’s possible for you to use INDEX to retrieve individual values, or entire rows and columns.
These values can be numbers, cell references, ranges, arrays, and constants, in any combination.
SUM can handle up to 255 individual arguments.
