I was fortunate enough to pick up an aggregate for my spindle for practically nothing. It will aid immensely in the manufacture of one of the pieces I make. It will allow me to skip 2 rotations and re-fixturing (Is that a word?) of parts.

I was looking at this video and saw the aggregate rotating. And this would save me a ton of work, and I thought about how to make one, mostly because the challenge seems interesting to me.
video

The thing I don't understand is how they are holding it in position. It looks like they are using a planetary drive with a belt to rotate the aggregate, I would guess the planetary along with the holding force of the motor is enough to hold position, but wouldn't the belt stretching give a degree or 2 of rotation? Or are belts that strong?

How do you think this was made?