UKC

so my alpine up has the option to not lock and according to the doc's thereby provide a dynamic belay. I can't think of any time when trad multipitching where a dynamic belay would be appropriate. A search of the internet gives plenty of guidance on how to give a dynamic catch but absolutely no guidance on when such would be appropriate. Can anyone suggest why I might want to use the dynamic capability over the auto-locking belay?

And before we get down the kN force applied to the top bit of gear rabbit hole, serious people have discussed this and the conclusion appears to be that other factors (rope stretch and body belay movement) mean that there is no particular threat of breaking the gear placement except where it is a snow belay anchor.

I ask however because in a series of quite big falls recently with over 30 meters of rope out a cam did get stuck in. I was tied down so NO body movement, auto-lock on, perhaps that caused the cam to get stuck in. But still a 3 0.25 factor fall shouldn't have exerted 2kN would it? so even an ATC wouldn't have slipped.

But still I can't see why anyone would belay in trad with the dynamic position on the Alpine up. Though I do recognise that tie down might not have been a great idea.

I thought some testing by Jim Titt, or perhaps someone else showed that the breaking force in the dynamic mode was poorer than a lot of manual belay devices, I don't use mine in the dynamic mode.

I agree that the general consensus from the posts I've read was that in reality using it in the clicked-up mode wouldn't increase the impact force on the top piece of gear significantly.

1) If your belayer is falling from a headwall and you want the ability to not smack them into the lip of the roof below.

2) Any situation where the fall zone is pretty clean but the wall is vertical or thereabouts and you want to give a soft catch so the leader doesn't hit the wall so hard.

There are a lot of situations on multi-pitch routes where you are standing on a comfortable ledge and if you know 1 or 2 might be a thing on the next pitch, you might give the belayer a bit of slack from the anchor so they can move a little more like when belaying from the ground.

As a climber you can feel the difference between the abruptness of the catch in the two modes - if you can feel it so does a marginal top piece.

What do you mean by a "cam got stuck in" and what has it got to do with belaying?

Having autolock on and a leash preventing body movement, and perhaps on the second fall the knot on the leader had tighter removed a lot of the opportunities for softening the catch. The highest bit of gear on one of the ropes was a cam, probably not very well placed, so when the gear was loaded it rotated and  one of the lobes got caught on a 2mm edge. Everything got jammed up and I couldn't retract the lobes to get the cam out. 

Would this have happened if I had been using dynamic belay option? Who knows but it did raise a question as to whether I was accountable for the lost gear. If my belaying was defective then I'd have thought so ..... 

It was a fall from a headwall which might have indicated a dynamic belay but actually there was no danger of hitting the lower wall as the the route was very overhung.

It feels to me that dynamic belaying was not called for and the leader should have aligned the cam with the fall line better, and the rope was out a long way indicating that the rope stretch was adequate in itself to protect the top placement. 

I strongly suspect that losing the cam was a result of the limitations of the placement or how it was placed rather than anything to do with the belaying.

I took an unexpected fall of a route around a year ago, and due to its unexpected nature and my belayers position/weight discrepancy I got a dynamic catch. Fell a little further than expected but it was nice and soft. When I tried the crux a second time I also fell, this time my belayer anticipated the fall and gave me the hardest of catches, smashing me abruptly into the lower wall and leaving my a bit bashed up. So I guess in some instances there is merit to giving a soft catch on trad 

I think there is some confusion about the two distinct types of "soft catch"

One is when the belayer leaves extra slack in the system. This will result in the leader swinging in to the rock more slowly but will put a greater load on the top runner.

The second is when the belayer steps in. This will result in the leader swinging in more slowly and, with good timing, put less load on the top runner.

So the first will sometimes be inadvisable for trad and the second often impractical unless belaying from the ground (when the possibility of runners "unzipping" will need to be considered anyway)

I prefer having the slack between my belay hand and my belay device in order to be able to control the amount and the speed of rope going to the falling leader.