No, that's not what the sort index does. You expect that the index labels a row with it's position. So given this data:
1
3
5
2
4

you would expect

1 0
3 2
5 4
2 1
4 3

But, in fact, the index wave for a given row gives the index of the data point that would be in that row after sorting. With Igor's index wave, you can get a sorted version of the wave this way:

Duplicate mywave, index, sorted
MakeIndex mywave, index
sorted = mywave[index[p]]

The index for my example above would look like this:
0 After sorting, the data point at row 0 would be in the first position
3 After sorting the data point at row 3 would be in the second position
1 After sorting, the data point at row 1 would be in the third position
4 Etc....
2

John Weeks
WaveMetrics, Inc.
support@wavemetrics.com