This card forces the target player to draw mana from lands if they are untapped, but that player can choose how to draw the mana if a land he or she controls has multiple mana abilities, or mana abilities with choices.

Since this is a Sorcery, your opponent may use Instants and abilities of permanents in response to this spell before you get the mana from their mana pool and lands.

If a land can draw a variable amount of mana, the target player (not the player of this spell) chooses how much to draw.