rotary vise?

thanks for all the response to my vise post. i do plan to keep doing this for as long as i am able, nothing smaller than a 12 probably. exactly what does a rotary vise do and why is if of benefit?
(please overlook my ignorance in this matter, i am self-taught and know no other folks that do this stuff)

THANX....

Here's a youtube video that demonstrates rotary tying.

http://www.youtube.com/watch?v=3b4dQ...eature=related

I just searched for rotary tying and picked on of the top choices. There are probably others.