Tap all lands target player controls and that player loses all unspent mana.
Since this is an instant, your opponent may use instants in response to the casting of this spell. Note that such spells and abilities will be resolved before the mana pool is emptied by the Mana Short. This makes it useless as a countermeasure for spells. To use it effectively, you need to use it during Upkeep and even then it will not prevent the use of instant spells and abilities.