You screw them onto the bolt with the plain metal end. The plastic gripper goes on last and prevents them vibrating loose, so often used on machinery.
After they have been used once, you are not supposed to use them again because the plastic part is deformed when you tighten them down.
The torque settings are not the same as for a plain nut, and if you use them again, will be different each time. Depending what you are using them for, this may be acceptable.
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