SQL Server OPENQUERY vs Regular Query against AS400

I log into a SQL Server database which has an AS400 server as a linked server.

I've discovered that running queries against this AS400 server is MUCH faster if I run the select in an OPENQUERY...

SELECT  EMEQP#
FROM _
WHERE EMALOC = '467' AND EMDLCD = 'A' AND EMSTAT NOT IN ('S', 'J', 'V')

Execution time: 46 seconds

SELECT EMEQP#
FROM
OPENQUERY(_, '
SELECT  EMEQP#
FROM _
WHERE EMALOC = ''467'' AND EMDLCD = ''A'' AND EMSTAT NOT IN (''S'', ''J'', ''V'')
')

Execution time: 1 second

Can someone explain or link me to why/how this is happening?

When you use openquery the query itself executed on remote server and you receive only results. In case of linked server, you local server does all the job. You may want to check
http://social.msdn.microsoft.com/Forums/eu/transactsql/thread/0e68f512-1e19-4c50-b343-219085d70076

StackExchange Database Administrators Q#33091, answer score: 5