Here’s the thing: You are paying a mana, a card and two life to get rid of one card in hand, for which the opponent has not paid anything (most of the time). There’s even a strong possibility that the card is dead later in the game or even harmful, if you only gain information by paying all those costs. How can this possibly be beneficial?

Actually, that’s not even the only cost. Often you pay extra life for fetches or shocks to be able to use the spell on turn one, nevermind the opportunity cost to make sure you are able to do this. Yet, as of this writing, according to MtGGoldfish, Thoughtseize is the 10th most played card in Modern, 31st most played card in Legacy and the 2nd most played card in Pioneer. If you look at the Arena only formats, the second-most played card in Explorer (they don’t give us data on Historic and Timeless). Admittedly, most of my lists start with 4 of them before even thinking about anything else.

So, why is it so popular? Why does, for example, Jund play it?

The philosophy behind Jund and other Rock decks is that you exchange resources in a way where you can hopefully gain small edges until you can gain enough of those edges to win. Fatal Push or Lightning Bolt, for example, can gain you a tempo edge by being cheaper than many of the creatures they can kill. Kolaghan's Command is fairly expensive for older formats, but it is usually a two-for-one as all of the modes are in themselves worth a card in many situations, so it can gain you an edge in cards even if you sometimes have to work to make it work for you. Tarmogoyf costs only two mana, but is often a 5/6 for that mana and is thus often worth quite a bit on the battlefield. Dark Confidant exchanges life for cards. That’s a pretty clear advantage to you.

Yes, my references are quite old, but these are all easily grokable.

But what about that Thoughtseize? You are not gaining mana. You are actually losing in mana by default. You are not gaining card advantage. It’s neutral in that regard and can be worse, if the opponent has no cards in hand or only has lands. It is negative in life total. You do gain information, but based on how little Peek sees play, that’s not generally seen as being worth a whole mana. (Gitaxian Probe did see a lot of play before being ousted from civilized society, but it does have the benefit of costing no mana and filling the graveyard for various effects.)

The thing is that many of the big boys playing Thoughtseize actually often cut them when sideboarding. They are just not good in many matchups. Having had the misfortune of playing 8-Rack against Dredge in the finals of a WMCQ, I know very well firsthand that sometimes it can be actively bad. So, what do you need for it to be good?

1) Disrupt a curve. This is a way to earn back the mana you just lost. If your opponent has a clean curve of 1, 2, 3, 4 in their hand, just take out the part that leaves them the mostly badly behind and what you can answer while optimizing your own mana and cards. If you can exchange one of your mana to even two of your opponents this way (meaning they can’t use mana on a certain turn), you can get ahead.

2) Disrupt synergies. Figure out which cards you’ll lose to and take the one that is hardest to find again. You can base this on how many copies are left in the deck, are there replacements and what is easy to find through tutors. Alternatively, if it’s early game and it seems like the opponent can’t play a specific card yet, take the one that is playable in the near future. This can be especially good, if you have other discard effects available.

3) Protect your plan. Want to play a combopiece? Check opponents hand first for counters or answers. You have limited number of threats in your deck? Again, check for counters and answers just before you play it. In these cases, you should often keep your discard in hand until right before you play whatever you are about to play, so that you don’t give an opportunity for your opponent to draw an answer later.

4) Get rid of otherwise hard to interact with threats. If you don’t have removal for a specific type of card, get rid of that. If your opponent could get value from a planeswalker before you can remove it, why not take that? There could also be creatures that can dodge your removal, so they can be good targets as well.

5) Get rid of card advantage. If your plan is to exchange resources, this is the most important target for your discard. The problem is that this doesn’t really work these days, when so many cards can produce value in different ways.

Uusally you have an idea when you construct your deck. You put the card into your deck or sideboard for a reason. At the same time, Thoughtseize is also flexible. You might have put it into your deck to protect your combo, but you can still use it to stop your opponent from comboing. Or you might have it in your deck to disrupt opponents curve, but you might also want to keep it in your hand until just before you play your big threat.

The role of Thoughtseize has changed in recent years. It used to be midrange attrition tool, but since those decks are dead, the card is now used for disrupting your opponent’s combo than simple exchange of resources. Still, it can play that role in some matchups depending on your deck.

One thing you always need to keep in mind is your own hand and deck. How does it match against what you see? You now have the information and use it to your advantage. Sculpt your game plan based on that. Play the threats that your opponents can’t answer first, or if you see a Fatal Push, play the threat you don’t mind dying first to hopefully entice an answer. Or, if you know your opponent, you might bluff that you have something more important to answer as well. This might be difficult, but players love outsmarting other players, so if you can let your opponent believe you have something more meaningful in your hand, go with it. Just remember that your opponents do often want to use all of their mana each turn and that might force their hand regarding removal.