This week was about countering the opponent. Remember that the main reason to scout (scouting was last week's topic) is to create the chance for you to counter the opponent.
Professor Feng said it's pretty obvious which units counter each other, so there's no need for him to cover all those specifics. There is actually a lot more to say about countering than just listing specific unit counters though. For example, let's say the opponent has 12 dragoons. What would be a good counter? A standard answer is speed upgraded zealots, maybe 14 of them. But then the professor showed us a replay of a match showing this exact situation where the *dragoons* win. In this case, the player controlling dragoons used an extreme amount of micromanagement to dance them around so that even the speed-upgraded zealots had to take lots of damage trying to reach any particular dragoon. After a bit of this, the dragoons get a lead and their superior number spirals to make the fight easier and easier for them.
So even though speed zealots would usually counter, the opponent can use micromanagement to survive the counter. What can yo do against an enemy who does this? The answer is to force him to run out of "attention" resource. Micromanaging the dragoons is kind of hard in itself, but there's no way he can do that while also defending against some other attack (or two!) in other parts of the map.
The next concept is the effects of scale on counters. The two examples were mutalisks versus marines+medics and vultures versus dragoons. Marines+medics are a "counter" to mutalisks, but only when the battle is of a certain scale. If we reduce the number of units on both sides to be very small, then mutalisks actually have the advantage. When each side has a fairly solid number, then m+m becomes the counter. And if we built an unreasonable number of units on both sides (just to test, not so much for a real match) we'd see that mutalisks again have the advantage. That's because if each side has like 100 units, the mutalisks can all stack on top of each other to concentrate firepower (their "flux") but the marines will be spread out.
The vulture vs. dragoon example is similar. Vultures with mines are a nice answer to dragoons, but if we have tons of units on both sides, there simply isn't room to lay enough mines for the counter to work. (Again, this is an issue of "flux.")
Apparently, extremely advanced pro Teran players sometimes build battlecruisers for this exact reason. When the scale of a battle is high enough, they simply can't get enough flux going with a zillion marines, even though they can build them. Instead they build battlecruisers just so they can concentrate the killing power into a smaller space. We were cautioned that we all suck too much to do this and it's mostly an issue for high level play.
Pendulum of Counters
Let's look at Terran vs. Zerg. When the game starts, each side has 4 units that can attack: 4 SCVs vs. 4 Drones. During this very early time, the Terran has the advantage. SCVs are stronger than drones. But soon, this shifts because the zerg player can have zerglings. For a little while, this gives him map control and an attacking advantage. Soon after that, the Terran player can have marines+academy (leading to medics) and this gives the Terran a strong advantage. But soon after that, the Zerg player can choose to have either mutalisks or lurkers. Either way, that gives him map control back.
Next, the Terran could counter mutalisks with either science vessels or with A LOT of marines+medics. Or he could counter lurkers with tanks. (Even unsieged tanks outrange lurkers.) But the advantage shifts yet again when Zerg gets either defilers+dark swarm or possibly guardians. This stage is very difficult for Terrans to counter, but building A LOT of science vessels is the main move here. Zerg then regain the advantage with either a) defilers + plague, b) queen + ensnare, or c) ultralisks upgraded to 5/3. Maybe the Terran can try firebats and siege tanks here.
The point of all this is that counters happen back and forth throughout the course of a game. This brings us back to scouting because you really need to know whether you or the opponent has the advantage at any given moment. If it's the opponent, you better keep your troops out of trouble and get going on developing your own counters. But if you currently have the advantage, you better press it while you can or your window of opportunity will soon close.
Then we saw a match where one player tries to trick the other about how the pendulum of counters is swinging. The Zerg player's overlord reaches the Terran's expansion area, but gets chased off by some marines. The overlord retreats but stays just close enough to see the minerals themselves. He can safely observe whether the mineral patches are being mined from here. The Terran player then does NOT build an expansion, and instead invests in an army. The trickery is that he sends some SCVs from his other base to mine those minerals to FAKE that he has an expansion here. This makes the Zerg player think he is safe for a while, but then the Terran players shows up with marines, medics, and firebats and dominates the Zerg's base, leading directly to the Zerg's forfeit.
Next Lore showed us a match he played last week. It was Zerg versus Terran. The Terran went with a 2 barracks + expand build. The relevant part of this match was when Lore built mutalisks and the pendulum of power was in his favor. During this time, his harassment tactics kept the Terran contained while Lore expanded to two more bases so he could get tons of gas. He knew the opponent wanted to build science vessels to shift the power back to the Terran side, but Lore managed to destroy the SCV that was building the science facility! Even more amazingly, he managed to destroy the next SCV that tried to finish building it...and the next...and even a fourth one, all with mutalisk harassment. The Terran player then sent a lot of marines to the area to make sure the science facility would finish building. This delay was critical though. By delaying the opponent's counter (science vessels), it allowed Lore to have free reign of the map for too long. By the time the vessels came out to counter the mutalisks, Lore had enough gas for a fleet of ultralisks, and that's gg.
A Stubborn Match
Compare this to a replay of another match of Terran vs. Zerg. In this one, the Terran player scouts to find that the Zerg has no attacking units at all, and hasn't even built a spawning pool yet. The Terran expands, because he safely can. (Perhaps he should have just attacked?) Anyway, the Zerg player rushed to mutalisks as fast as he could, and established map control with them. He actually had really great micromanagement skill with these mutas and managed to destroy a ton of turrets, many marines, and many SCVs. So far, he was doing well by containing, expanding, and doing as much damage as he could with the mutalisks. But the Terran was able to build science vessels--a lot of them. At this point, we know the pendulum of power has shifted because the Terran has the counter. Strangely, the Zerg player STILL built more mutalisks and was determined to try to win with them. The vessels irradiated muta after muta and started to dominate the match. Even though the Zerg player continued to pull of nice micromanagement with the mutas, he was fighting uphill against a counter for far too long. The game dragged on for a long time, and eventually the Terran won.
In another example game, this time Protoss vs. Zerg, the Zerg player builds a spawning pool and gets scouted. Perhaps zerglings are coming...but he actually builds none. By the time the Protoss player realizes this, the Zerg has a lair and spire going up. Meanwhile, the Protoss player was building a stargate so that he could make corsairs. Corsairs are great in that they can kill the Zerg's overlords and they can also scout (and the Protoss player *badly* needed to scout now...the Zerg really had nothing but an illusion so far). Because the Zerg player rushed to spire so quickly, skipping all attacking units, he was able to build a pair of scourge before the Protoss player's stargate even finished! A hovering pair of scourge over the stargate kind of premtively counters the Protoss player from scouting. This move kept the Zerg player's actual weakness (no army!) secret long enough from him to switch his full economy into full unit production.
Forcing a Counter
The last bit involved some semi-fake games between the class administrators, but it didn't quite work out. The idea they were trying to convey though is that you might want to bait the enemy into building a specific counter so that you can be ready to counter that. Protoss built corsairs against Zerg, for the reasons already covered (kills overlords and allows for great scouting). The Protoss expected and *wanted* the Zerg player to build hydralisks to counter. The Protoss's true goal here was actually to build lots of zealots and hope that the opponent made lots of hydras. Zealots are extremely strong against hydralisks, and it apparently takes 26 hits(!!) for a hydra to kill a zealot. Tricky to get your opponent to build just what you want, but apparently by choosing the right build order yourself, it is reasonable to this off in a match.