Button E is a button on the Mach 5, it controls special illumination lights "which can be controlled singularly or in tandem", allowing the driver to see more clearly than with ordinary headlights. When used with the “night shades” attached to Speed’s helmet, his vision is enhanced with infrared light. The original Japanese version strictly translates this as "illuminating eyes".
Button E was later modified to activate mini-wings that would slide out from under the car to assist Speed in long jumps.