It is simple and mostly just extends the previous game prototype a bit to explore invoking feelings of rebelliousness.
"key" means physical keyboard key. You're supposed to find out you can press down to jump. and doing so is supposed to feel rebellious, since it's not part of the main instructions or necessary for the main goal. As it says in the intro text, there are a couple other motivators to feel rebellious.
Also the circle in the top right is the secondary goal. Once you find out you can jump, that may be more obvious.
But initial testing shows no sign of the rebellious emotion being invoked. Perhaps this prototype is more of a proof that the internal narrative of the player is required to be "just so" before they can feel this class of emotions.
http://vazor222.com/MushroomLeap/ (requires flash)