If you trace the `UPDATE STATISTICS T WITH SAMPLE 50 PERCENT` in Profiler you see 


    SELECT StatMan([SC0], [SB0000])
    FROM   (SELECT TOP 100 PERCENT [SC0],
                                   step_direction([SC0]) OVER (ORDER BY NULL) AS [SB0000]
            FROM   (SELECT [C] AS [SC0]
                    FROM   [dbo].[T] TABLESAMPLE SYSTEM (5.000000e+001 PERCENT) 
    				WITH (READUNCOMMITTED)) AS _MS_UPDSTATS_TBL_HELPER
            ORDER  BY [SC0],
                      [SB0000]) AS _MS_UPDSTATS_TBL
    OPTION (MAXDOP 1) 

With Plan

![enter image description here][1]

`TABLESAMPLE` [is documented here][2]

> `TABLESAMPLE SYSTEM` returns an approximate percentage of rows and
> generates a random value for each physical 8-KB page in the table.
> Based on the random value for a page and the percentage specified in
> the query, a page is either included in the sample or excluded. Each
> page that is included returns all rows in the sample result set.

That source also states 

> Although the plan shows a table scan is performed, only those pages
> that are included in the result set are actually required to be read
> from the data file.

The `STATMAN` call is to an internal aggregate function [briefly described here][3]


  [1]: https://i.sstatic.net/SVwg7.png
  [2]: http://msdn.microsoft.com/en-us/library/ms189108%28v=sql.105%29.aspx
  [3]: http://www.sqlskills.com/blogs/conor/statistics-damned-lies-and-statistics-what-is-statman/