(Why) Should I hold the rope when belaying the second climber with Reverso?

Question

When climbing a multi-pitch route, suppose I am belaying the second climber, using Reverso or a similar device.

It lets me arrange the rope in such a way that when the climber falls, the rope automatically tightens, and the fall is stopped without the belayer's intervention (see picture).

However, some people say that I need to hold the rope anyway.

What is the reason for this requirement? I could come up with the following explanations:

1. If I arrange the rope in a wrong direction, holding the rope will prevent slippage
2. Holding the rope prevents me from getting distracted
3. If I am not holding the rope, it makes me responsible for the outcome of any unrelated disaster
4. It's a generalization of the principle "don't let go of the rope", which is true for sport top-roping

However, none of these seems reasonable.

Answer 1

All of the listed reasons hold true, and can largely be simplified to the fourth: never let go of the brake strand unless you have tied it off. This maintains good habits and also mitigates potential accidents. In the past, such devices have been considered hands-free if monitored (i.e. you're sitting next to it, the ropes are running cleanly, and you're just grabbing a sandwich or a drink). It may be that more conservative approaches will develop over time, but this is one case where there still appears to be a general tolerance for taking a hand off. (Whether the manufacturer's instructions agree with this is a different question.)

A Reverso (or any similar device) used in guide mode has some potential failure modes:

The first failure mode to watch for is when the rope does not run in line with the brake strand. E.g. if the anchor is offset from the last piece of protection, when the rope comes tight in a fall it might run across the side of the device instead of over the brake strand. Result: the brake strand is not captured and no braking occurs.

A second potential failure mode is for the loaded strand to "pop" under the brake strand or jam alongside it. The latter case is not immediately dangerous, but will require work to unbind the strands. The former case will result in very little braking power, which is a Bad Thing.

These cases can be mitigated against by making sure the ropes are running in the correct direction and that the rope diameters are well within the sizes the devices are rated for.

Largely pulled from: https://www.mountainproject.com/v/atc-guide/106838345__1

Answer 2

Technically there shouldn't be a problem letting go of it. The conditions to this are that the setup is done correctly and there is no unforeseen failure.

In the event of a failure there is not much that you holding onto the end of the rope could do to help control/save the situation, but who knows.

Below is a link to question regarding the strength of atc guide in autolock mode.

What is the strength of an ATC belay device when used in a guide mode/multi-pitch setup?