Problems understanding ARC and zombies

[ramy1989]

I started learning Cocoa when there still was the manual counting, but after few time I got the new OS update and ARC was available.
I know how to user retain-release if I don't use ARC, but the problem comes with ARC enabled.I always used strong references and everything was fine, never had leaks nor dangling pointers.
But I want to understand better how it works.

Now I am running a project for debugging, with zombies enabled.I tried do do this:

    __unsafe_unretained NSString* str;
    @autoreleasepool
    {
        str= @"Hello";
    }
    NSLog(@"%@",str);

I would except that str is zombie because @"Hello" should be autoreleased, and I print it outside the pool.But instead it just prints "Hello" and all goes fine.Why?

Also a tip, should I start making some projects with manual counting to understand how ARC works?