It is mostly for downward pressure on the rest with the arrow allowing a more repeatable shot across the rest.I found that looping with a normal loop that there CAN be some twist associated with it and yes I did tie in a nockset above and below.
I talked to a coach yesterday and he said that the Louisville shoot was loaded with pros shooting the loop under.He also likes this method better than a standard loop.
It' funny,I made the switch 4-5 years ago and havn't looked back.
Braden addressed this issue briefly some time ago on his forum on his web sight.He also said it really is a matter of preference.