just like mentioned above..it enguages first gear and reverse...
which locks it in place..everything is loaded inside the trans..sans the ringear
you could consider it like chaining your car to a tree..puting it in first gear and pulling the chain.....the chain holds you there...
if you drop the chain and the car launches....its that basic pricipal......

you can put what ever amount of rpm you want at the stage...
its only based/limited by your converters stall rpm..

if you have a fast car..have your head/neck ready
it can suprise you..
chepast