The users of this power are able to become incorporeal, being capable of passing through all solid matter.
The natural state of ghosts is to be intangible. While in their normal form, ghosts have no effect on solid matter and flicker when a person or object passed through them.
While in their spirit form, ghosts are able to partially affect people and objects that they pass through, knocking them around.
All Shadow Demons are said to possess this ability; though their entry in the Book of Shadows describes the power as phase-shifting and specifies that it involves "Rearranging the atoms of any object or person they touch." Parker Caine possesses this ability while in his shadow form.
Solid matter is usually undamaged by a Shadow Demon passing through it. However, when phasing through the Satyr Leon to retrieve a piece of the Scythe of Tartarus, Parker caused significant damage. Leon was so injured he barely made it to the Vera Manor, and he was left with a mark where Parker had phased through him, even after being healed by a Whitelighter. However this may be a consequence of having a shard of the Scythe pulled out of one's body, as when phasing through the Taweret Sela she seemed to experience no injury.
A Shadow Demon can selectively phase, keeping part of their form solid while making another part intangible. They can also phase a solid object or person they are touching, as Parker did when pulling Sela's pendant through her body, or phasing Maggie Vera through the attic floor of Vera Manor.