Phase rock areas are slightly out of sync with reality.
Effect: While in a phase rock square, a creature is insubstantial and phasing. When a creature starts its turn in a phase rock square and moves out of that square, it is insubstantial and phasing until the end of its move.
Usage: Use phase rock to let creatures that don’t normally have phasing move through blocking terrain or obstacles, allowing them to avoid defenders and surprise the characters. Be careful with placing too many phase rock squares, because creatures can remain insubstantial in those squares, taking half damage and prolonging encounters.
Published in Manual of the Planes, page(s) 22.