Birdcage, etc
A birdcage occurs when a wire is unlayed -- the outside strands will open and expand outward. The wire looses a lot of strength, the outside strands do not take the load, and everything is transferred to the heart.
I have never seen a wire fail due to the lay being tightened enough to break strands, but it could happen.
Most wire in furlers is damaged from the jamming the furler (usually with halyard wrap). It can happen on the way in - winching the furling line, or on the way out -- winching the sheets.
If things are jammed the lay will tighten in one direction, and unlay in the other -- in and out with the sail, depending on the wrap direction.
I have seen a torsion rope used in the luff of free flying sails -- I think Yale makes it. It would take the question of lay out of the equation.
Bob
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