Having experimented with this method (fireman's using a belay device) over the years, a few thoughts: Managing slack/tension is inherently slower than with classic (i.e. no devices) technique. Time is of the essence. There is no faster way to remove slack and tension the rope than by simply using two hands on the rope. It's an expert technique which non-experts tend to try to copy, ineffectively. A non-expert cannot properly focus on the rappeller, as attention is diverted to managing the belay device. Looks flashy, but is less effective than two hands on the rope. Mobility is hampered by being attached to the rope, making the belayer more vulnerable to falling objects. Falling over backwards can be risky: you might effectively tension the rope, or you might swing into the dirt/water with the falling guy landing on top of you a la Wile E. Coyote. Attempting to climb the rope is probably a better last ditch. Best to stick with classic technique: two hands on the rope, no devices. It's effective, KISS and easily teachable. There is no faster way to remove slack and tension the rope than by simply using two hands on the rope.