Historical Rules · 26 August 2016
Rule 901.11
Official source text from the 26 August 2016 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.