Drain Power

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

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 they control has multiple mana abilities, or mana abilities with choices.

Your opponent may use instants and abilities of permanents in response to this spell before you get the mana.

A mana ability is an ability that (1) isn't a loyalty ability, (2) doesn't target, and (3) could add mana when it resolves.