Why is it acceptable to belay from above with a grigri but not using the same movement from below?

Question

A common practice is to belay a follower from above by connecting a grigri to the anchor and pulling down on the brake strand to take up slack.

However, if you were to do this while belaying from below (holding the brake strand upwards, parallel to the climber strand at all times) you would get yelled at for unsafe belaying.

While belaying from below, you must keep the rope in an S-curve through the device as often as possible:

Why is one acceptable and the other is not?

Answer 1

I think it's more a question of positioning than anything.

If you're belaying from below with your arm/hand above, in the event of a fall, the main muscle working on putting tension on the cam is the deltoid (with a little trap). It's relatively weak in stopping the shoulder from rotating if something pulls on your hand (high moment over the entire arm lever) so there's a higher chance you might not be able to stop the slippage from occuring before it's too late.

When belaying from above with the brake line below, your arm is strongest with your lats and triceps contributing significantly to apply tension on the cam so it would engage very quickly.

Answer 2

Specifically for a Grigri, there is nothing immediately unsafe about belaying like this as the camming action of the device is what holds a climber in a fall. The issue (apart from the comfort of the belayer) is that it inspires bad technique through muscle memory when belaying with devices that purely rely on friction (such as a standard ATC). There is also the incredibly slim chance the Grigri will fail, in which case it will revert to a standard friction device.

In either case, belaying straight up like that will not introduce any friction into the system and the device will not hold a fall.

There is also an argument that we have a collective responsibility to others on the crag, and this sets a bad example to novice climbers who may see this technique without appreciating the difference between an assisted braking device and a normal friction device.

Answer 3

There are two different things going on here- belaying a follower from above is a toprope scenario- the amount of rope is only going to get shorter as they climb up, all a belayer needs to do is to pull rope through.

In your second image, you show lead climbing, we keep our hand below the belay device so that we're in a position where we can feed rope back through the belay device as the leader needs additional rope.

The third image is from the youtuber Hard is Easy, who is not producing content on behalf of petzl. Petzl documentation never makes use of this S-curve, and never suggests that it be used. The correct setup for a grigri is to have the brake strand running over the side plate of the grigri.