To play Jello, all you need to do is to let your body naturally sway with the movements of the car.
My informant told me about this game while we were sitting in a car full of people. She told me she had learned about it from her best friend while they were riding the bus. After talking about it, we started to play Jello. I noticed while we were playing that there is a competitive nature to the game; the people in the car would also use the force of the car movements to powerfully shove people really hard to one side of the car.
This game was probably developed as a way to make a car ride more interesting and fun as it gave passengers something to do. At the same time, a typical car ride is filled with a lot of movement. From my experience, whenever I sat in a full car, I would be constantly leaning and bumping into the people sitting next to me every time the car turned. Either, I or the other passengers would be constantly apologizing for invading each others personal spaces. By playing Jello, this awkwardness is eliminated as it is completely okay to lean on other people in order to participate.