Bob Purcell (Chief Threadologist of Superior Threads) discusses S and Z twist and micro strands that make up the final product. Having a higher twist-per-meter on a thread results in having a smoother, stronger, less lint thread.
An incorrect twist direction causes twist displacement in the sewing thread which leads to excessive thread wear and sewing problems. Twist displacement, and the accompanying unwinding of the thread, is caused when the thread is pulled through the needle eye in an unfavourable feeding direction (friction on the edge of the needle eye). In normal operation this can be observed during reverse feeding.