In a Limited event, each card a player opened (in a Sealed Deck event) or drafted (in a Booster Draft event) is their sideboard.

You must cast the sideboard card you wish to cast while Command the Chaff is resolving. You can’t cast it later in the turn.

If a spell has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

If you cast a card “without paying its mana cost,” you can’t choose to cast it for any alternative costs. You can, however, pay additional costs. If the card has any mandatory additional costs, such as that of Tormenting Voice, you must pay those to cast the card.

You can’t play land cards from that sideboard.