When you select from a table on an indexed column and order by that column in the opposite direction of the index, does the index help
If the index is covering it's likely it will be used in conjuction with a sort operation in the query plan to reverse the order.

Edit: Following a little more thought!

It will depend on whether this is a trivial query or involves joins. If trivial:

SELECT x,y FROM MyTable ORDER BY y DESC


and the index order is Y ASC, a reverse scan of the index leaf level should avoid a sort. 

If non-trivial:

SELECT x,y FROM MyTable mt INNER JOIN MyOtherTable mot ON mot.y = mt.y


it should depend on the sort order of MyOtherTable.y. If it's ASC as per MyTable.y then the two indexes would be read in index order and a sort applied after the join. If it's desc, in theory a reverse order index scan could be used for the join and an additional sort wouldn't be required to satisfy your order by clause.

Edit2: Couldn't recall if this would show up in the execution plan in SQL Server. The icon doesn't indicate this is a reverse scan, nor does the tooltip on hover. Properties however shows 'Scan Direction - Backward' or checking the plan XML reveals

&lt;IndexScan Ordered="true" ---&gt;ScanDirection="BACKWARD"&lt;--- ForcedIndex="false" NoExpandHint="false"&gt;
            &lt;DefinedValues&gt;
              &lt;DefinedValue&gt;
                &lt;ColumnReference Database="[TestDb]" Schema="[dbo]" Table="[MyTable]" Column="OtherId" /&gt;
              &lt;/DefinedValue&gt;
            &lt;/DefinedValues&gt;
            &lt;Object Database="[TestDb]" Schema="[dbo]" Table="[MyTable]" Index="[IX_MyTable_OtherId]" /&gt;
          &lt;/IndexScan&gt;