Marty and Doc escaping the police force in 1985-A in order to get to 1967. The best part is that due to a wire getting damaged, they can't get fast enough by flying. Doc's solution? Putting the DeLorean into a nosedive and gunning it, hitting the 88 mph just before they crashed into warehouse or got shot by a police officer with grenade launcher.
Doc: Fasten your seat belt, Marty. We've got one chance... Marty: [fastens his belt] What's that? Doc: Gravity! [DMC pitches downward]