When a character's limbs or body is held in place by objects or devices that are made of rigid materials and locked into position such that they will not budge or sway if the victim struggles.
Key examples are pillories or stocks mounted in place, shackles bolted or screwed to a surface, (such as a wall, floor, or table) cuffs holding the subject inside a sex machine, and pipe arrangements built around the subject. Restraints that are tethered by ropes or chains do not count, nor do tentacles. (organic or metallic)
The victim may have some freedom of movement or ropes or chains in addition, but at least one restraint must not move or sway in response to the captive's struggling and convey a sense of being solid.
In some cases the restraints themselves may be able to move, such as a machine capable of posing the victim, but the restrained subject must not have any influence upon the restraints.