Drain Power

• WWizards of the Coast2004-10-04
  Official Ruling
  Your opponent may use instants and abilities of permanents in response to this spell before you get the mana.
• WWizards of the Coast2004-10-04
  Official Ruling
  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.
• WWizards of the Coast2004-10-04
  Official Ruling
  If a land can draw a variable amount of mana, the target player (not the player of this spell) chooses how much to draw.
• WWizards of the Coast2013-04-15
  Official Ruling
  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.

Rulings via Scryfall