You simply have the ability to say no to whatever your opponent tries to do while burying them in card advantage
But a statement by committee member Jim Lapage suggests that the committee instigated the change
Not too many notes, but heres what we have: If you may cast spells for an alternative cost (say, with Fist of Suns), you may use that alternative cost to Jump-Start a spell, but still have to discard

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Once a creature has been blocked, its considered blocked until the end of combat, even if it gains unblockable