Flashback, a keyword ability that appears on instant and sorceries, is making its third trip back to Magic expansions tomorrow when Innistrad is released. To get you prepared for the prerelease here are some general rules on how Flashback works.
Flashback is actually two different static abilities. The first creates an alternate play cost and method. Flashback allows you to cast a card from your graveyard by paying the Flashback cost as opposed to the card’s mana cost. For example, you can play Ancient Grudge as normal from your hand by paying one red mana. However it Ancient Grudge is in your graveyard you can play it via its Flashback ability by paying one green mana. The flashback cost of a card is usually different from the cards mana cost, but not always. And as shown in this example the flashback cost can be a different color from the mana cost. Even if a spell is cast via its flashback cost, the color of the spell is determined by its mana cost. This Ancient Grudge will be a red spell even if it is played via flashback.
The other static ability is applied when a spell that was played via a flashback cost is on the stack. The effect from this ability prevents players from overly abusing flashback cards. When a card is cast via its flashback cost, anytime it would leave the stack it is exiled as opposed to going to a graveyard. This happens if it resolves, if it is countered or leaves the stack in any way. In other words, you only get one chance to replay a spell via flashback.