True twist:
After the true twist of the fiber bundles, the outer fibers of the bundles are twisted by inclined spiral threads, the fibers are twisted and deformed, and the yarns are tightly embraced, which changes the structure and mechanical and physical properties of the collective fibers.
When the enclosure angle exists, the fibers have centripetal pressure on the bundles. The larger the enclosure angle is, the greater the centripetal pressure is. Because of the existence of centripetal pressure, the outer fiber is extruded into the inner layer, which increases the tightness of the yarn and the friction between the fibers, thus changing the structure and physical and mechanical properties of the yarn, which is the essence of true twist yarn.
False twist:
False twist yarn is also often used in textile fabric design. Hold the two ends of the multifilament and twist it through twisting device in the middle of the two ends of the multifilament. The twist figure of the opposite twist direction and the same number of twists can be obtained at both ends of the twisting device: S twist at one end and Z twist at the other end. The sum of twists of the whole yarn is 0. Generally speaking, the two ends of a rope are fixed and twisted in the middle. The tighter one side is twisted, the tighter the other side is twisted, but the opposite direction is twisted. Therefore, the total twist on the rope is unchanged, still 0, so the added twist is called false twist.