What is "Rhystic" Magic?
The Prophecy expansion features "rhystic" cards - cards taht have some effect unless a player pays to keep the effect from happening. For example, Withdraw is an instant that reads, "Return target creature to its owner's hand. Then return another target creature to its owner's hand unless its controller pays ." When Withdraw resolves, the targeted creature's controller may pay to stop the second part of Withdraw's effect from happening.
As with all spells and abilities, most choices (other than targets and modes) are made on resolution, not when the spell or ability is played. For rhystic spells and abilities, this includes the choice of whether to pay the cost to stop the "unless" part of the effect.
When a rhystic spell or ability resolves, its controller carries out its instructions in the order they're written, just like any other spell or ability. When it's time for the "unless" clause to resolve, the specified player chooses whether to pay the cost to stop it. (Only mana abilities can be played at this time.) If he or she does, that part of the effect is canceled. Otherwise, it resolves normally.
Notice that although the affected player doesn't have to decide whether to pay the cost until the spell or ability resolves, he or she will already know what targets have been chosen for it.
Example: Chitra plays Withdraw. She puts it on the stack, chooses as its target two creatures Jamal controls, and then pays its mana cost. Later, when it resolves (assuming it hasn't been countered), Chitra carries out the instructions in order. First, she returns the first targeted creature to Jamal's hand. Then, before she returns the second one to his hand, Jamal chooses to pay 1 to stop the rest of the effect from happening. The second creature remains in play.
Also check the Prophecy FAQ for information.