Its a good question. In a normal automatic transmission putting the tranmission in "Park" engages a pin into the gears to prevent them from moving. The pin is called the "Parking Pawl". Its not as robust as most people think.

For one thing delivery people are sometimes are in a hurry and they slam the tranny into Park while still rolling a bit. This eventually causes the Pawl to fail. I've seen it happen. While attenting at college in the UP of MI, I ordered pizza from such a person. He parked on a particular street in Houghton, MI called Garnet Street and came inside. Wwhile giving me my pizza I watched over his shoulder as his car shuddered twice and then slipped out of park rolling down the hill onto some tennis courts (through a chain link fence). Yes I tried to warn him it was about to go... but was too late.

As to your original question..... I'm not entirely sure whats in there, but I'd guess its very similiar to a parking pawl and is just a pin sticking into the gears to stop them. The tranny still has gears, they are just configured completely different than a normal transmission. From the one cutaway diagram I saw its a planetary arrangement but I'll be ****ed if I know how it works, I'm an EE not an ME.