The parking brake mechanism on an auto tranny works by a metal rod (the parking brake pawl) moving out into a cylinder with recesses in it (windows) and falling into one of the recesses and thus preventing the car from rolling. This setup is towards the rear of the tranny and effectively locks the output shaft stationary in relation to the transmission housing. This is why the cars rolls a little bit, the car and output shaft of the tranny have to move before the pawl can find the next recess to fall into.
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record