Historical Rules · 28 April 2017
Rule 901.11
Official source text from the 28 April 2017 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 701.22.