Auctioneer's Discretion The auctioneer has the right at his or her absolute and sole discretion to refuse any bid, to advance the bidding in such a manner as he or she may decide, to withdraw any lot, and in the case of error or dispute, and whether during or after the sale, to determine the successful bidder, to continue the bidding, to cancel the sale or to reoffer and resell the item in dispute
Put simply, "split second" means "As long as this spell is on the stack, players can't cast spells or activate abilities that aren't mana abilities." The keyword was introduced in Time Spiral (one of my favorite sets of all time)
The differences here can be subtle, so if you are not sure, go back to the light test and the edge check
Rainbow Rare Trainers tend not to be too pricey amongst TCG cards, so it will be interesting to see where they land based on their association with the Pokemon GO games
Its an over-statted 3-drop, a strong 5-drop, and a threat that needs an answer so you dont win the game
Either way, you'll need to save up Pack Points by regularly opening packs, earning five points at a time