Question : Need some help understanding security filtering within group policy

I'm having a problem getting my GPO to work. It's a GPO to make only internal computers to access the WSUS server. Here's what I've done so far:
1. I've created an OU container in AD called 'SUS GPO'
2. I've created a group in AD called 'Internal Computers' that contain, for now, a test computer
3. I've created the GPO for WSUS and it works fine with security filtering for 'Authenticated Users' with the test machine
4. If I remove the 'Authenticated Users' and add in the group 'Internal Computers' it no longer works.

My questions are: What's the difference between using Security Filtering than to say have my Internal Computer group within the SUS GPO container that is already linked to the SUS Policy? If the policy is linked to my OU and within my OU there is the group Internal Computers that have my test computer in it, where does security filtering come in?
2nd question is: why is it not working now that I removed 'Authenticated Users' from security filtering and added the 'Internal Computers' group?

Thanks experts,
B

Answer : Need some help understanding security filtering within group policy

The tutorials are in the Microsoft SQL Server Books Online. The tutorials (which use the AdventureWorks database) can be found here: http://msdn.microsoft.com/en-us/library/ms167593.aspx