Don't fall on teeth, because I did it for you
.Toothed devices are intended to be progress captures up a climbing rope or in a complex pulley system. If you get a high enough force, it can desheath your rope and possibly cut through it. Petzl says that happens at 4kN and we did some human testing to find out if that is true.
Understand WHERE The Risk Is
There are two (becoming more common) use cases where people are actually climbing up a rock while using micro traxions as belay devices. In top rope soloing you have the rope prefixed to the anchor up top and you slide the micro traxion up as you climb. In belaying a follower, you may use this rather than an atc in guide mode or a grigri as those can wear your arms out. If you are new to belaying, DON'T use this as a belay device until you are proficient at the other methods, have experience in what happens in the real world, and can very quickly convert it to a lowering system, in an emergency, as the teeth only let the rope go in one direction.
You may be attentive to belaying your partner up to you until the last few feet because the last section is a little easier, they are right there, they want to get to the anchor so they go a lot faster all of a sudden, but this is where all the risk is. You can fall 10 feet on the micro traxion.... IF YOU HAVE A LOT OF ROPE IN THE SYSTEM. If you are standing on a ledge at the anchor and you have 5 feet of slack in the system, you have a HUGE chance of cutting through your rope if you fall, even if it doesn't "feel" as dangerous.
I was "top roping" on a 9.8 Boa Dynamic Climbing rope being belayed by Sylvester Jakubowski with a Petzl Micro Traxion, which he donated new for all of our human and lab testing. I had about 50 feet of rope in the system and fell 10, 15, and then 20 feet and got 1.97kN, 2.71kN and 3.45kN which was not enough to desheath the rope.
Fall factor is how far you fall compared to how much rope you have in the system. If I want more force, I need to fall further with the same amount of rope or go up higher and fall the same amount with less rope. Since that is where the risk actually is, short falls near the anchor, we did our next tests up higher.
My first fall was not even 10 feet but I generated 3.24kN which was the same as my biggest fall further down. My 2nd "bigger" fall was maybe only 15 feet and I got 4.5kN. My rope did NOT desheath at 4kN like Petzl says in their warnings. Shortfalls near the anchor, similar to an aggressive bounce test on a big wall, produced up to 3kN. Even if I don't desheath it at 4kN, your body starts to hurt past that so we did the rest of the tests at the lab.
We used a 200lb weight on the drop tower and tested the Nano and Micro on Dynamic and Static rope. On the slow pull machine we did the same but also a 9.2 polyester static canyon rope, and 6mm accessory cord. We used an ascender to compare because it has a longer tooth section like the pro traxion.
Pro, Micro and Nano are the three options by Petzl. Protraxion is 5x heavier than the nano but it's also way way easier to haul with. The micro and pro both have a locking feature to keep the toothed cam permanently up while using it as a pulley only or installing a rope.
Nano's are intended more for super light weight mountaineering. Micro is better for belaying or top rope soloing and can also be used if hauling 75lbs or less on a big wall. Pro traxion is better for big heavy hauls.
We did a follow up video where we tested a rope man with less aggressive teeth and a device similar to a rescucender with a ribbed cam and the rope man desheathed the ropes the same and the ribbed cam went a little higher before desheathing a rope. See us testing this Wildcountry Ropeman1 groved cam in this Instagram POST.
Shockloading near your anchor is a huge risk of high forces. We also did some human testing to find out what really happens.