Kasoke-tai ridatsu (幽体離脱 Astral Projection) is an ability that manifests in someone who feels the need to be in more than one place more than twice in their lifetime. It's rare, though it seems to manifest easier in humans than in spiritual beings such as Shinigami.
Astral Projection allows a user to project their consciousness out into a astral form of themselves. When a user first gains this power, their body slumps to the floor as if dead when they use it. After sufficient training, a user can stay conscious in astral form, effectively making the astral body function as a clone. An astral form of the user is always tangible with it is first formed, but it can become intangible if the user wishes, making them able to walk through walls with it, or have sword cuts fade through them with no damage to the astral form whatsoever. If the user possesses a Zanpakutō, they are almost always unable to use it while in their astral state. For an unexperienced user, the astral projection is exhausting.