I don't know exactly how it works because I hate automatics and don't really give a rip about how they work, but...

...all automatics should have the parking brake set BEFORE putting the transmission into park. Every automotive manual I have ever read even states this. "Park" locks the transmission by engaging some sort of fork or such into gear teeth. If the vehicle moves the fork or whatever it is will lock tightly against the teeth and that's why it is difficult to disengage. If it locks tight enough you have a good chance of removing a gear tooth while disengaging, which is BAD!

I've heard of many people having to get tow trucks because they locked their park so tight they couldn't even get it disengaged. All you automatic folks do yourself a favor and always put the parking brake on FIRST. Once you get intot he habit you won't even notice you're doing it. Strangely, despite never owning an automatic, this is stuck in my head and so I do it when I drive other people's cars.