RKF, or Rhythmic Kicking Firehead, is a shooter mixed with a rhythm game. You don't actually have to do anything in time with anything - the game automatically fires for you, and its firing sets off drum sounds. An ever-increasingly intense wall of enemies comes towards you, which you must shoot to eradicate.
Use your mouse to aim.
A context menu should be shown when right-clicking or, on touch devices, long-pressing in the game. This menu especially allows to play in full screen and control the volume.