Historical Rules · 17 June 2011
Rule 901.10
Official source text from the 17 June 2011 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 701.20.