If Elminster is untapped and does not have summoning sickness, you may tap Elminster as you cast a spell from your hand. If you do, then you control an opponent of your choice while casting that spell. While you control that player in this way, that player can only activate mana abilities of lands they control, and only if that mana is sufficient to pay the cost of that spell or if that mana is spent on the cost of other mana abilities that are played in this way. You can spend that mana to cast that spell, but you cannot spend any other mana on the cost of that spell.
This is strange and have to change it to make it work. See also: Word of Command