It’s a decent little Enchantment that’s 1U (one colorless and one blue) to cast. Whenever an opponent shuffles his or her library, you can look at the top two cards of their deck. You may exile one of those cards and put the other (or both) back on top of the deck. The best part about this effect is that you get a choice of what to remove or put back. It is a sort of “fate sealer,” as well. If you’re milling your opponent, then chances are they are running out of their key cards that they can beat you with. This gives you an opportunity, albeit only when your opponent shuffles, to remove a key card from the game. It also allows you to know what card or cards they may draw next.
Ordinarily, this card would not be that great, and a bit under-powered for a rare. At the time, it was argued that this card was more deserving of an uncommon slot. But that ability to choose what cards get exiled or go back on top is what makes this good. Obviously, if you have two or three of these in play you can repeat the process for as many triggers as you get to truly muck up your opponent’s game. It does need a deck that is primarily focused on being a blue-black control deck that relies on mill as a win condition; otherwise, it’s just taking up space.
Still, Wizards seems to be doing all that they can to make mill a serious Modern contender for competitive game-play. With all of the shuffling from fetch-lands that goes on in that format, Psychic Surgery could prove to be a part of that strategy, even if only as a sideboard option.