All of the cards they tapped from the previous round are untapped
For the franchise's 25th anniversary in 2021, there were 50 cards you could pull -- all 24 Starters plus Pikachu as both holo and non-holo cards
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Not surprisingly, the card hasn't been reprinted since 1993 (the very first set of Magic's existence), although there was a Mystery Booster card printed in 2019 called Problematic Volcano which effectively serves as an homage to this card, and according to Ethan Fleischer, a mechanic that worked in a similar vein was experimented with for Theros: Beyond Death in 2020
Will's deck will cast mega-bomb cards like Storm Herd and Finale of Glory with ease this way, but the deck isn't 100% reliant on Will for lifegain to pay off
(2011-01-22) "Flashback [cost]" means "You may cast this card from your graveyard by paying [cost] rather than paying its mana cost" and "If the flashback cost was paid, exile this card instead of putting it anywhere else any time it would leave the stack." (2021-03-19) You must still follow any timing restrictions and permissions, including those based on the card's type