Post #3, top left photo. The swivel arm with adjustable stop/clamp.
It is intended to swivel to maintain the radius while the courses go up.
With two guys building, it was too slow and if not removed very carefully would disturb the brick just set while trying to get to the next one...
We mixed the mortar more stiff and worked quickly between batches taking full advantage of the wood wedges to maintain profile until they set.
Just my experience, mos builders love them.