Theoretically, if there was a short to ground, a voltage potential could develop between the two legs if the driver took a step away from the car. By hopping and landing with feet together, both legs will always be at the same potential so no current should run through the legs even if the immediate area around the car is energized.

It looks absurd, but it's standard advice if a backhoe or crane hits a power line.

https://youtu.be/Xvlk_73bSvc?t=2m31s

And why you should keep your feet together when there is a thunderstorm. An impact near you can still kill you if your feet are apart.

It means the driver is never part of a conductive path between the car and the ground.

Or between different parts of the "ground" that are at different potentials.