For 1R (1 red mana, 1 colorless mana), Impact Resonance is an Instant spell that allows you to deal X damage to any number of target creatures. In this case, X is the greatest amount of damage dealt by a source to a permanent or player this turn. In a multiplayer format like Commander where plenty of damage is being dealt all the time, playing a card like this to take out any number of problem creatures can be highly useful. The damage doesn't have to even be caused by you in the first place. With this card, you could have really powerful mass removal in your hands.
So if this card can be so destructive at such a low mana cost, why does no one play it? Truthfully, there are many cards like this that in theory are quite good. But in the current Commander meta, Impact Resonance is a bit too situational to be given a deck slot where it would be used consistently. It's a well-designed card, but while it's a really sweet trick, it just isn't going to impact games often enough to be worth it. It's more of a cheap trick, although it can make for some major game changes. This obviously is why it was designed in the first place.
The only popular Commander who would seem to want this sort of effect is Heartless Hidetsugu, who is notorious for his extremely damaging ability. His ability deals damage to half of each player's life total (rounded down). This card would be a nice way to clear the board of creatures that may pose a problem to the Heartless Hidetsugu player. It's not an auto-include, but in playgroups that like creature-heavy decks, I can see it being a useful "tech" card.
What do you think of Impact Resonance?