While ~ is in the stack and has been successfully cast, all player's hands are revealed, and you can cast cards from any player's hand if that spell targets this spell, and you can also activate abilities of permanents any player controls if those abilities target this spell. You can spend mana from that player's mana pool as well as your own mana pool for the cost of that spell or ability. While doing so, you can activate mana abilities of permanents that player controls, if the resulting mana is spent on the cost of other mana abilities of permanents that player controls and/or on the cost of the spell or ability.
