Rear louver

From Futurepedia, the Back to the Future Wiki

The rear louvers light at initial thrust.

The rear louvers of the DeLorean time machine were constructed to cool the nuclear reactor that the flux capacitor used to generate 1.21 gigawatts of electricity. Later, after the DeLorean's hover conversion, they were used to propel the DeLorean while in flight. When they were operating, they lit up to a bright white color. After the initial acceleration, the louvers would cease glowing.

A similar series of louvers were used on the Jules Verne Train for propulsion.