1) Open the console
2) Type in 'tf_taunt_first_person 1'

3) Type in 'cl_first_person_uses_world_model 1'

Now with these enabled, join any server with bumper cars or a halloween server. First, you will notice you can see your body. Next, the fun part. Getting in a bumper car! :D

Have fun getting seeecret advantages (if it even is one).
I recommend Spclans cause they have a bumper cars command.