The machines are computer-controlled hands that tap (or spank, if you prefer) the car to the programmed beat. As if that weren't enough, two pairs of free-standing hands, with arms, clap time at either end of the car. If there's a meaning to it, we're not aware of it, but it's fascinating to watch.
It isn't often – actually, never – that we get to say, "Ladies and gentlemen, we present The Spankers." You'll find them below, and the action begins in the first video at the 2:40 mark.