When life totals are exchanged, each player gains or loses the amount of life necessary to equal the other player’s previous life total. Replacement effects may modify these gains and losses, and triggered abilities may trigger on them. A player who can’t gain life can’t be given a higher life total this way, and a player who can’t lose life can’t be given a lower life total this way (see rules 119.7–8).
April 27, 2018 · Dominaria
If a creature fights itself, it deals damage to itself equal to twice its power.
September 30, 2016 · Kaladesh
Some effects instruct a player to “play” with a certain aspect of the game changed, such as “Play with the top card of your library revealed.” “Play” in this sense means to play the Magic game.
February 1, 2014 · Born of the Gods
Neither activating an ability that creates a regeneration shield nor casting a spell that creates a regeneration shield is the same as regenerating a permanent. Effects that say that a permanent can’t be regenerated don’t prevent such abilities from being activated or such spells from being cast; rather, they prevent regeneration shields from having any effect.
September 30, 2011 · Innistrad
Neither activating an ability that creates a regeneration shield nor casting a spell that creates a regeneration shield is the same as regenerating a permanent. Effects that say that a permanent can't be regenerated don't prevent such abilities from being activated or such spells from being cast; rather, they prevent regeneration shields from having any effect.