In reply to gethin_allen:
Sorry for not getting back sooner, I have been out climbing alot. (nice weather in spain and south france).
To explain in more detail?
The problem is NOT when using a sling in the anchor, it is when belaying over the ring of a anchor (as a beginner would do).
purplemonkeyelephant: love the drawing, it shows what i am trying to say.
When loaded, the pivot will go to the right or left in a fall, this will move the pivot about 45 to 60 degrees.
The sideways force just under axle is ( i think) to much for that fin bit of material?
I will try and make a photo, next time i am in the same situation.