It's done by bypassing the clutch so that the car starts w/o you pressing on it.
My friend got one installed on his accord a few years back. One day he let me borrow his car so my girl and I drove around in it for a few hours. When I got out the car I left it in 1st and went inside my mother-in-laws appt. Me being the cool guy I tried starting the car from inside of her appartment. Her mom starts creaming: it's coming in! it's coming in! when I turned and looked out the window the car was coming towards the appt. I noticed that the car stopped right before it hit the wall, so I ran outside and surprisingly the car was still running, and the only thing stopping it from crashing into the wall w/ the 3 glass windows was 2 small bushes. Just to think about all of the damage it could of caused... makes me wanna cry! lol so yeah it could be dangerous.