FORCESEEK and FORCESCAN

FORCESEEK

FORCESEEK [ (index_value(index_column_name [ ,... n ] )) ]

Use the FORCESEEK hint to Specify the query optimizer use only an index seek operation.

SELECT ..... FROM Sales.SalesOrderHeader h

INNER JOIN Sales.SalesOrderDetail AS d WITH (FORCESEEK)

ON h.SalesOrderID = d.SalesOrderID .......

SELECT ..... FROM Sales.SalesOrderHeader h

INNER JOIN Sales.SalesOrderDetail AS d WITH (FORCESEEK (INDEX_NAME ((COLUMN_NAME))) 

ON h.SalesOrderID = d.SalesOrderID .......

FORCESCAN

Use the FORCESCAN hint to force the query optimizer to perform a scan operation

SELECT ..... FROM Sales.SalesOrderHeader h

INNER JOIN Sales.SalesOrderDetail AS d WITH (FORCESCAN) 

ON h.SalesOrderID = d.SalesOrderID .......

http://msdn.microsoft.com/en-us/library/ms187373.aspx