In reply to TobyA:
I think it is the ridges that do it.
They are wide and narrow (if you see what I mean). This means even the larger nuts can be placed in narrowish slots and have lots of the ridged surface in contact with any wee rugosities that will then jam them in. That and pretty floppy wires mean they can't be pulled out and a key is almost always needed.
Not sure why nuts need to be ridged at all....surely in a fall the downward force on the wedge creates outward force thus jamming the nut in place harder an harder. Ridges are pretty much superfluous to this action, no?
I only used them a few times on rough dolerite BTW.
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