Historical Rules · 1 July 2012
Rule 901.8
Official source text from the 1 July 2012 Rules release
Planechase games have an inherent triggered ability known as the "planeswalking ability." The full text of this ability is "Whenever you roll {PW}, planeswalk." (See rule 701.21, "Planeswalk.") This ability has no source and is controlled by the player whose planar die roll caused it to trigger. This is an exception to rule 112.8.