The first time Isaac takes damage while in an uncleared room, time is stopped for 2 seconds and Isaac becomes a ghost and leaves his body behind, which drops to the floor. While in ghost form, Isaac has flight and spectral tears and the next hit he takes is negated.
Isaac's tears and speed are greatly increased while time is stopped, although this is not shown on the HUD.
Isaac's ghost re-enters his body after the room is cleared or if he gets hit again.
The damage blocked by being in ghost form also affects self-damage and red-heart draining sources, like devil beggars, IV Bag, etc.
Receiving lethal damage can potentially save your life if you manage to heal yourself before the ghost reenters the body.
Astral Projection can also save The Lost from death; however, it does not regenerate the Holy Mantle and kills you when the room is clear.
This can be avoided if The Lost gains a Soul Heart before the ghost reenters the body.
Dull Razor may be used to safely activate Astral Projection as The Lost.
It's also possible to leave the room while the ghost is outside the body, either by bombing it or having items that keep doors open, such as Door Stop or Mercurius. This grants you an additional chance to heal.
Teleporting outside of the room will not work.
Astral Projection cannot be triggered in Boss Rush.
Hypercoagulation: Due to the effective speed up, the heart used to activate Astral Projection can easily be collected before it disappears.
Old Bandage: When getting hit with only one heart left, Astral Projection allows for picking up the dropped heart in the brief time of invincibility even when the player has no hearts left. Failing to do so or having no hearts dropped results in guaranteed death however, but with luck this loop can be used to potentially save runs.
Holy Mantle: The shield will activate before Astral Projection.
Plan C: Will instantly kill Isaac without triggering the ghost effect.
Spirit Shackles: This item will not activate if Isaac received lethal damage when Astral Projection activated.
Rock Bottom: Does not preserve the tears and speed bonus Isaac receives after activating Astral Projection.
The item description is a reference to Guillotine.
Astral projection is an esoteric belief where the "astral body" is separated from the physical body and can travel throughout the universe.
If Astral Projection is activated during the Dogma fight, after defeating its second form, Isaac will die during the transition to The Beast fight.