Decks must be randomized at the start of every game and whenever an instruction requires it. Randomization is defined as bringing the deck to a state where no player can have any information regarding the order or position of cards in any portion of the deck.
The phrase “order or position” is key. A deck that is “mana-weaved” is not random; even though the player may have no information about the location of a specific card, they have information about the cards’ order (that is, land-spell-spell, land-spell-spell). Also key is that a player shouldn’t know any information on where a card is, not even which half of the deck it is in.
Pile shuffling alone is not sufficiently random and may not be performed other than once each at the beginning of a game to count the cards in the deck.
Pile shuffling is completely non-random, since individual cards can be tracked and since cards are shuffled into a deterministic order. A single pile shuffle can help players count their decks and loosen sticky cards, but more than that a pile shuffle does not contribute to randomization and will qualify as Slow Play. Once the game has begun the need to count the deck during randomization is largely gone. As such, a single pile shuffle at the start of the game is permitted, but is not allowed at any other time. Please remember when applying the IPG that habits are hard to break, and a single caution may be appropriate the first time.
Once the deck is randomized, it must be presented to an opponent. By this action, players state that their decks are legal and randomized. The opponent may then shuffle it additionally. Cards and sleeves must not be in danger of being damaged during this process.
The deck should be fully randomized when presented to the opponent, and the opponents’ shuffles are not meant to make the deck “more random.” The purpose of the second shuffle is to discourage deck-stacking and cheating.
If the opponent does not believe the player made a reasonable effort to randomize their deck, the opponent must notify a judge.
Judges still need to know what’s going on. Earlier versions of the IPG included an example under Cheating which outlawed “3 pile shuffling to undo a suspected mana-weave,” but this example was removed in the February 2015 update. While not calling a judge may no longer be cheating, judges should still be notified immediately.
Players may request to have a judge shuffle their cards rather than the opponent; this request will be honored only at a judge’s discretion.
Judges should use their discretion on this. Shuffling takes a significant amount of time, during which the judge can’t take other calls. Shuffling for players should be reserved for exceptional circumstances.
If a player has had the opportunity to see any of the card faces of the deck being shuffled, the deck is no longer considered randomized and must be randomized again.
At Competitive and Professional Rules Enforcement Level, a player who does not re-randomize their deck after seeing cards has committed Looking at Extra Cards and may need to be investigated for cheating.
At Competitive and Professional Rules Enforcement Level tournaments, players are required to shuffle their opponents’ decks after their owners have shuffled them. The Head Judge can require this at Regular Rules Enforcement Level tournaments as well.
In some cultures, asking to shuffle an opponent’s deck would be considered an insult. The requirement to shuffle an opponent’s deck is intended to prevent cheaters from taking advantage of these cultural norms.
Because a deck presented to an opponent should be random, this additional shuffle does not need to be thorough. However, simply cutting the deck is often not sufficient to meet this requirement.