Shimenawa (注連縄 enclosing rope) are strands of rice straw rope used for ritual purification in Shinto, usually decorated with shide (紙垂), zigzag-shaped paper streamers. It is believed that something adorned or surrounded with shimenawa is a sacred location or object.
Shimenawa in Touhou
It is prominent in Kanako Yasaka's design. Not only is it worn around her neck, wrists, waist, and on her feet as part of her sandals, but also as a large, circular version on her back. This represents an entwined snake such as in an ouroboros, symbolizing resurrection, rebirth and eternity. Shimenawa also adorn the onbashira she prefers using.
In Silent Sinner in Blue, it is explained by Watatsuki no Toyohime that shimenawa was created by the lunarians as a way to bind and imprison the impure and even gods. Using this, they sealed Ookuninushi to the Izumo Grand Shrine to prevent him from taking revenge on them for taking his land. It is then implied that Ookuninushi's son, Takeminakata-no-kami, was bound to the Moriya Shrine to prevent the same.
- Wikipedia's entry for Shimenawa