Imagination Realization. As the name suggests, it is the ability to manifest anything you can imagine.
It is the ability possessed by elementals, which are the sense of touch of the natural world. By connecting their will directly to the world, they are able to transfigure the world into whatever environment they imagine. Of course, the level of manifestation varies depending on the scale of the elemental, and it goes without saying that only nature can be transfigured—things that exist independently of nature are immune.
Also, in the world of Tsukihime spiritual realms are not alternate dimensions, but rather exist within the world itself. Things like legendary lands and countries of eternal spring are the products of the imagination of whatever elemental rules over them. Consequently, it is possible for humans to wander into them by accident. The castle in the mountains that Arcueid calls home is actually a product of her marble phantasm.
In Melty Blood, it makes an appearance as Arcueid’s Arc Drive. Countless chains act as an extension of her sense of touch, temporarily augmenting the sensation in her hands. As for why the organ doing the grabbing is a chain instead of hands, it’s probably because of her feelings about always having to restrain herself.

MB Act Cadenza PS2 Manual: Tsukihime Dictionary