Well I think that originally the band was being used to accomodate resistance. It was never mentioned in any of the videos. But if you will notice, instead of the swing arm attachment like most reverse hypers...they have the weight on a chain to make the movement more difficult. Just an example of Westside trying new things.
In the Seminar video which is the newest you can see them using the same reverse hyper machine...and the band is not there anymore. Perhaps they tried it and didn't get much out of it. You could always ask Dave at his website...http://www.elitefts.com
Remember nothing is static in the Westside system. Everything is changing...go to keep the progress going.