Sealed Format is a Deck format used at Sneak Peek events.
In this format, players are given unopened packs and make Decks with a minimum of 20 cards, using only cards they get in these packs.
The usual Deck-build rules don't apply here. A player can have as many copies of a card as he/she likes in their Main Deck, Side Deck or Extra Deck if they get more than 3 copies. Also, all cards that you don't use in your Deck are used as your Side Deck.
In Sealed Pack Play, at the start of a Duel, each player can choose to place their entire hand on the bottom of their Deck, then re-draw 5 cards. Each player can only choose to do this once per Match. The player who goes first has the opportunity to do so first, then the player who goes second has this opportunity; if the first player decides not to re-draw, they cannot re-draw even if their opponent decides to.
Prior to the rule change that prevented the player who goes first from conducting their normal draw during their first turn, this was required to be performed before the first player conducted their normal draw.