2.2. Game Play Error — Looking at Extra Cards
Definition
A player looks at a card they were not entitled to see. Players are considered to have looked at a card when they have been able to observe the face of a hidden card, or when a card is moved any significant amount from a deck, but before it touches the other cards in their hand. This includes errors of dexterity or catching a play error before the card is placed into his or her hand. Once a card has been placed into his or her hand or if a player takes a game action after removing the card from the library, the offense is no longer Looking at Extra Cards.

A player is not considered to have looked at extra cards when he or she places a card face down on the table (without looking at the card) in an effort to count out cards he or she will draw.

This penalty is applied only once if one or more cards are seen in the same action or sequence of actions.

Examples
A. A player accidentally reveals (drops, flips over) a card while shuffling her opponent’s deck.
B. A player flips over an extra card while drawing from his deck.
C. A player sees the bottom card of her deck when presenting it to her opponent for cutting/shuffling.
D. A player activates a Sensei’s Divining Top that is no longer on the battlefield, and sees 3 cards before the
mistake is noticed.

Remedy
Penalty: Warning

Additional Remedy
Shuffle the randomized portion of the deck (which may include the cards that were seen, if they were part of the random portion of the library). This requires first determining whether any portion of the deck is non-random, such as cards that have been manipulated on the top or bottom of the library, and separating those. Once the deck has been shuffled, any manipulated cards are returned to their correct locations.

Care must be taken before shuffling to make sure that there are no “legally known” cards in the library. Check with both players to verify this, and check the graveyard, exile, and battlefield for deck manipulation cards, such as Brainstorm and cards with the scry mechanic.

Philosophy
A player can accidentally look at extra cards easily. Drawing extra cards is a separate, more severe Game Play Error.

Players should not be using this penalty to get a “free shuffle” or to attempt to shuffle away cards they don’t want to draw; doing so may be Unsporting Conduct — Cheating. Players also are not allowed to use this penalty as a stalling mechanism. The deck is already randomized, so shuffling in the revealed cards should not involve excessive effort.

When the life totals are exchanged, each player gains or loses the amount of life necessary to equal the other player's previous life total. For example, if Player A has 5 life and Player B has 3 life before the exchange, Player A will lose 2 life and Player B will gain 2 life. Replacement effects may modify these gains and losses, and triggered abilities may trigger on them.

If a player can't gain life, that player can't exchange life totals with a player with a higher life total. If a player can't lose life, that player can't exchange life totals with a player with a lower life total. In either of these cases, neither player's life total will change.

We both know how it works, just the idea is that instead of making it life gain and loss, why couldn't you just switch it and not have it register as gain or loss.

Jelly has it right. Setting a life total to a particular value works by having the player gain or lose the appropriate amount of life. This means that Erebos's ability effectively reads "It is impossible for your opponents' life totals to go up."

Since it's impossible for your opponent's life total to go up, the part of the exchange where their life total would go up is also impossible, and if part of an exchange is impossible, all of it's impossible, so nothing happens.

Can an evoke cost be reduced with Heartless Summoning? Is evoke an alternate casting cost?