On a shoe and bootmakers forum I read someone was having trouble with their threads unwinding at the slightest provocation. They knew the thread and code were good because both of them had been used by other people before.
The picture they posted showed that the thread had been plied up using the opposite twist than would normally be used, which made me realise that a left-hander might end up doing the same thing if they were mirroring instructions written by/for right-handers.
When I ply up a thread I twist it up by rolling it down my right thigh, away from my hip towards my knee, with my right hand resulting in a clockwise twist if you look at the end of the thread. If you rolled the same ply down your left thigh with your left hand you’d end up with an anti-clockwise twist to the thread, which won’t want to stay plied up because the resultant plied cord is an S/S twist instead of an S/Z twist and cords plied with the same twist as their strands won’t stay plied.
If you are left-handed and you make threads on your left thigh, you need to break the thread by rolling it up your thigh, knee-to-hip, and ply the cord the same way.
When you ply up a cord the twist of the ply has to be the opposite of the twist of the component strands, in other words the twist you ply the cord with should be the same as the twist you use to break the individual strands.
The reason for this is that if you ply with the opposite rotation from which the strands were spun, then the fibres in the final cord lay straight. If you ply with the same twist you are placing additional twist on the already-spun fibres, which is then going to naturally want to untwist.
The difference between an S-twist and a Z-twist can be a bit hard to explain, and google wasn’t vending with anything useful so I drew this picture: [update: It has been pointed out to me by someone who knows far more about fibre work and spinning than I do (thanks CW and AdW) that this picture is backwards. What is labelled S should be Z and vice-versa. I will fix it as soon as possible.]
I’ll follow this up with some pictures of S/Z and S/S plied cords that show the fibre directions which will hopefully clarify this explanation.