I have a bunch of these fittings in the flat in Edinburgh, and I have some observations...
We have mostly roller blinds, but one set of wrinkly-tights design, all sourced from JL just along the road in Embra.
The difference between them is this:
The roller blinds have u-shaped brackets at either end to accept the ends of the roller, and one or more of the butterfly twist-lock brackets mid-span to prevent the roller bowing downwards.
The vertical load is almost entirely taken by the more substantial u-brackets at the ends of the roller, and this is where the hauling of the cord is applied. There is almost no load on the mid-span twist-lock brackets. These are absolutely rock-solid.
The wrinkly-tights blind has no roller, and is entirely supported by twist-lock brackets, and is prone to getting pulled down, particularly if you haul on the cord at an angle outwards rather than vertically down.
I investigated it, and came to the following conclusions...
The twist-lock brackets can be good enough, if they are well-enough made and correctly installed. The ones on my wrinkly blind were utter crap. The mid-span ones on the roller blinds were actually much better made.
Installation: as others have said, they need to be in a straight line, and vertically pretty level, in order to correctly engage in the slot of the blind. There is a certain amount of tolerance that can be achieved by flexing the blind, but not much.
When fitting the blind to them, you need be shoving the blind up with a fair force to ensure both wings on the twist-lock bracket go fully through the slot in the blind, and engage inside it. A common failure is to have only one wing engaged. The cam lever should start out pointing out towards you, and then rotate by a full 90 degrees to be flush with the blind, and lock with a firm feel.
Construction: The construction and tolerances on my wrinkly blind were utter crap.
With the bracket off the wall in my hand, I observed the following: with the wings in their engaged position in the slot, there was still sufficient fore-and-aft play for the thing to disengage. Vertical pull was just about OK, but any pull in the outward direction would disengage the blind from the brackets. Just crap design. There should not be anything like that play available when the wings are engaged. I bodged some material into the blind to take up the free play in the locked position.
The second issue was that the wings were made of cheese, and were prone to bend downwards under modest load, thus making the twist-lock action very sloppy and not firm, and the corresponding friction between the bracket and blind was low, and this exacerbated the tendency for the blind to slip forward and off the front wing, as described above. I re-bent them so they sat horizontally for now at least.
But even now, I treat it very carefully, applying only gentle vertical pull on the cord.
