Weird experience at US airport security

Last year I travelled to the Ryder Cup in Minnesota (outcome very disappointing for Europe).

On arrival at Minneapolis airport for departure home I joined the queue for security checks along with my buddy, he was directed to the normal scanner search etc whilst upon looking at my passport I was directed differently and told "your pre-cleared", not knowing why or what this meant I made my way to the scanner to be told "no need, your cleared".  I had an iPad in my bag, charging cables, phone and some coins, I told the scanner team, and they reluctantly said place it on the conveyor.  When I started to remove my belt to go through the scan gate they said again, "your cleared, come on through".

Has anyone else (James Bond apart) experienced this or perhaps a security insider can explain.

Thanks

Joe