Essentially, you can sacrifice one of your card still in the deck in order to stop your opponent from doing any Special summoning for the whole turn or whatever else you need at that time
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Rather than pay its mana cost, you can exile a red card with mana value X from your hand
Of course the solution theyre working on may be more of a roundabout approach, involving hybrid mana or somesuch, but in any case I expect itll lead to more planeswalkers being playable in Standard, which is always good
The closest we got before was in a different MTG color, and easier to play, though (we think) less fun: the 6-cost Red sword Embercleave, from 2019's Throne of Eldraine set
These selections will show you which one they originate from