As you can see, I had some difficulty with this setup, pulling slack through the device. This was vastly improved by adding a locking carabiner between the grigri and the mallion. I'm pretty excited to use this system more in the future. To explain the system: The grigri is the primary belay, which is backed up by a mallion and backup knots that won't fit through the mallion (hopefully). I'm sure some people would be comfortable without the mallion and with backup knots that would block the grigri. I use the mallion because I would like a second attachment point to the rope (not just one locking carabiner to the grigri). ALSO, the mallion being lower than the grigri helps the grigri to lock! ... https://www.youtube.com/watch?v=L_HAynmEIuQ

... https://www.youtube.com/watch?v=87fWUh_zoFQ

Here is my preferred method of rappelling from a sport climb using a fireman's belay as a backup. The fireman's belay means the belayer holds both strands in tension, acting as another hand on the brake. That way if the climber cannot hold the brake, the belayer still holds the brake. It is possible to hold the climber up this way against his will, which makes for a good practical joke.

... https://www.youtube.com/watch?v=mrMxXwDLap8