How does the PARK of an automatic transmission work?

I ask because just recently when I put it into park the car rolls forward just a little by may be 10 or 20 degrees of a wheel turn, doesn't sound much but you feel it when you take your foot off the brake pedal.
Never used to do it. Just want to make sure its not a symptom of something else.

Brakes are fine, the car stops dead.

Thanks

 

I think that's fairly common. I can't remember ever driving/riding in an automatic that didn't roll forward a bit when put into park. I think it's because there is a bit leeway in the transmission gears such that gravity will pull the car forward or back until the gears meet and the car stops moving.That's just a guess though, I've never actually looked into what "park" does.

 

Yeah this is typical.

 

Yeah, Autos have a tab that locks it into Park. If it happens to not line up with the gear then it will roll a little until the tab catches. I was researching AT for the last few days to see how they work. There's some cool stuff on howstuffworks dotcom.