Someone who specializes in tying people up, for Bondage scenes.
Basically a person who hires themselves out, to properly tie another person up for a bondage scene, or BDSM play. However, they approach this as more of an art form, to show off the intricate patterns that can be obtained in tying a person up.
In many instances, this is more for the art benefit, for showing their craft up for photographic sessions, more than for any role playing or other associated activities.
Noteworthy: There is even a category for Bondage Riggers in the SIGNY Awards.
Should be noted, not all Riggers are actually participants, though some are. They are primarily interested in the art aspect of bondage, and how they can immobilize a person.