Untap all creatures you control during each other player's untap step.
All your creatures untap during each other player's untap step. You have no choice about what untaps.
Other effects can prevent a creature from untapping during an untap step. You do need to look carefully, however, as many effects say that the creature doesn't untap during its controller's untap step, and this card's ability applies during other players' untap steps. If a card does say this, then Drumbellower can untap it.
