As a little kid watching TOS reruns, I used to wonder about this. I thought maybe the balls were hollow covers that would perhaps rotate up into a hidden slot exposing the 'vents' for when the ship used it's warp engines.
Twently years wiser and...it's stock footage reused from the pilot
Yup, I thought pretty much the same thing as a kid. The spheres were used for orbital velocity, while the vents were seen when Enterprise sailed off at the end of the ep.