- The key characteristic of cards with proportional effects is that they have consume mana in two separate ways: the mana cost of the card itself, and the card's "Spend all your Mana" text. Playing a card with a proportional effect will therefore first consume mana equal to the cost of the card, before then consuming all remaining mana as per the card text.
- Because the proportional effect itself as part of the card text and not the base cost of the card, it only consumes mana if the card is successfully played. For example, if a Counterspell counters the casting of Forbidden Flame, no mana will be consumed.
- Effects which trigger based on the cost of a spell will count the cost of the card itself, not including any mana consumed by the proportional effect. For example, playing Forbidden Flame with Atiesh equipped will summon a 0-cost minion, even at 10 mana.
- Similarly, the mana cost of a minion with a proportional effect does not itself increase depending on the mana consumed: playing Forbidden Ancient at 10 mana will produce a 10/10 Ancient, but Evolve will still transform it into a random 2-cost minion, despite the card consuming 10 mana when played.
- After the mana cost of a card with a proportional effect has been paid, all the player's remaining mana will be consumed as part of its effect, and the card's effect will be increased in proportion. Thus, playing Forbidden Shaping with 10 available mana after an opponent's Loatheb would still leave the caster with 0 mana, but would only summon a 5 mana minion.
- Proportional effects always consume all the player's mana (up to a maximum of 10), even if it is not possible to produce an effect of that magnitude. For example, playing Forbidden Ritual with 10 available mana will consume 10 mana even though it can never summon more than 7 Icky Tentacles. Likewise, Forbidden Ritual can be played on a full board, with no minions summoned, but will still consume all available mana.
- The mana cost of a card with a proportional effect can be modified as usual by cost modification effects, but not below 0. Thus, a Sorcerer's Apprentice would not normally affect a Forbidden spell since it already costs 0 mana; however, it would act to partially counter an increase in cost due to an effect such as Loatheb. Cost modification effects cannot alter the proportional effect itself, since this does not have a cost but simply consumes all available mana.
- Proportional effect cards can be played even when there is no mana left to consume. For example, casting Forbidden Healing on any target when there is 0 mana will not produce any healing, but will still trigger a Violet Teacher's or a Wild Pyromancer's effect.
- Players can determine how much mana to spend on a proportional effect by expending mana in other ways first, leaving a smaller amount remaining for the proportional effect to consume.
- Proportional effect cards cannot be followed with the playing of any other cards in the same turn, unless they are 0-Mana cards (e.g. Moonfire, Target Dummy), or following regaining Mana through effects such as The Coin or Innervate. This makes it wise to carefully assess the desired sequence of events, make use of your Hero Power and possibly play other cards, before finally playing the proportional effect card.
- Proportional effects currently represent the only effects that consume the player's mana. Mana is normally consumed by the base cost of playing a card or using a Hero Power.
- 模板:Whispers of the Old Gods patch Added, with Forbidden Shaping, Forbidden Healing, Forbidden Ancient, Forbidden Ritual and Forbidden Flame.