This comes into play first in highlighting the car's aerodynamics. Few specifics are given, though. Nissan does say that aerodynamics were key in the design of the Leaf in order to make it more efficient and go farther, but also to make the car quieter. The company also says that the design was inspired by the shape of an airplane wing. We don't exactly see it. According to Nissan, the new Leaf is also lower than the previous model to reduce lift and make the car more stable.
The other part of the smoothness theme is apparently ride quality. Part of the teaser video implies smooth ride on road trips. We of course appreciate ride comfort, but we hope the car isn't so soft that it's wallowing and unpleasant in corners. We should know much more about all of these topics when the car is revealed on September 6.