Historical Rules · 27 March 2015
Rule 901.11
Official source text from the 27 March 2015 Rules release
After the game has started, if a player moves the top card of his or her planar deck off that planar deck and turns it face up, that player has “planeswalked.” Continuous effects with durations that last until a player planeswalks end. Abilities that trigger when a player planeswalks trigger. See rule 701.21.