This week's class was about how to use APM (actions per minute). In other words, each player can only click so many times per minute, so he must make best use of those clicks.
Professor Feng divided APM use into three categories:
1) Incompressible. These things you just HAVE to do. If you don't do them at the times you should, you still have to do them, just later on when you remember or have time. Examples are building your economy, responding to harassment, and moving your army.
2) Compressible. These things you can do at your leisure. If you have some free moments, you can spend all your clicks on these things. If you have more important things going on, you can let these slide. Examples are scouting, micromanagement, and harassing the enemy.
3) Extraordinary. These things are above and beyond the usual. Examples were proxy (which I thought I knew the meaning of, but not sure in this context) and deception (will be covered in a later class).
I think the most useful idea of this week's class was illustrated by a graph that I will describe in words, rather than draw for you. Think of a graph over time that shows how many clicks you are doing at each moment. First, imagine the clicks that you need to do that come from building workers. Every 20 time units, or whatever it is, you'll click because of that. This by itself is not a big burden. But soon you will have enough money to build a gateway or other building that makes units. From then on, you must add a new set of clicks for building attacking units. Later, you have enough money for a second gateway, and this adds a new set of clicks to build attackers from there. So as time goes on, you have a heavier and heavier burden of clicking until at some point, you can't do it all anymore and you must choose what to do.
There's more to this though. If we look at the graph, we see that at some points in time, lots of things demand your clicks all at once. Maybe the time you're supposed to build a new probe is right at the time you also should build a gateway and right at the time you also should build a zealot out of some other gateway. So there are some points in time where your available clicks are pretty much maxed out. And these clicks are all *incompressible*. You must do them, so if you are slow, it just means you're taking longer than you should to complete the necessary task.
There are other points in time where you don't have to do any of those clicks. Maybe all the lulls line up so that you have a few moments where you don't have to build a probe or a zealot or a gateway. You can then do some scouting or harassment or move your army to better position.
The key idea here, the most insightful part, is that it's extremely favorable if you can use your *compressible* actions (the ones you do while waiting around between the super-important stuff) to harass the enemy in hopes that he will fall behind on *incompressible* actions. The basis of this theory is that harassment is extremely powerful and demands a counter. If I send a reaver/shuttle to your mineral line, you can't really decide to just ignore that and keep doing your incompressible actions. My one reaver can completely devastate your economy so you MUST respond. And even if you can respond and can completely stop my reaver, you are obligated to do so, even if that means delaying all your incompressible clicks, like building more probes, more gateways, more zealots. Because I can initiate harass during my "free time" and not fall behind on my important clicks, I don't get behind as much as you, because chances are you will need to delay your important clicks.
You might ask, "what if I happen to harass the enemy during his compressible time, so he doesn't get much behind either?" Sure that is possible, but remember that you are the one who initiated the harass, so you can choose to do this at the times most favorable to you, and chances are you'll catch the opponent at a bad time for him, at some point.
To summarize in one sentence: harassment lets you use your compressible APM to delay the opponent's incompressible clicks.
Next, we saw some examples where expert players manage to do compressible and incompressible actions "at the same time." Flying a shuttle or corsairs around the enemy base to harass are examples of compressible actions that you can do when you have free time. Some replays showed that experts sometimes give the shuttle (or corsairs) an order of where to fly, then during travel time, they do incompressible actions. So while the shuttle is flying from A to B, even though that might take only 1 second, they use that time to build more probes, zealots, etc, right on time.
Another example showed a Terran player who knows that a reaver drop is coming. When it does come, he knows that he will be obligated to do many clicks to stop it, and that this might put him behind. Before the drop happens, he uses some of his compressible action time (the time when he can do whatever) to put up a few turrets and have vultures lay a few mines in smart places. When the reaver drop did come, the Terran hardly had to click at all to stop the harass. By investing his earlier compressible actions in this way, he didn't need to get behind on incompressible actions when the reaver drop came.
Harassment is such a key part of APM that we looked more closely at it. There are many things could make a unit good at harassment: speed, flying, powerful attacks, splash damage, or annoying things like cloaking. But the Professor highlighted speed as the number one thing and flying as a good number two.
Harassing with high speed units has two strong effects. One is that you can use these units to go out of your base and attack something...and then quickly return if they need to defend. If your harassment units are faster than the enemy's main attacking units, you harass with them without worrying so much about being defenseless at home.
More important than that though is that fast units mean the enemy must be able to defend at EVERY base. If they have a group of units to defend against your harass, you probably force them to split that group in half (one half at each base). Your fast units can go to one base, and simply leave and go to the other if the enemy defends only one.
Flying units have a similar concept in that the enemy must defend not just one entrance to a base, but EVERY entrance. They must even defend against cliffs that ground units cannot traverse.
Comedy Harass by Boxer
Boxer usually plays Terran, but below is a video of him playing Zerg against a poor Protoss player. Boxer harasses with zerglings in a hilarious way. Note that his force of zergling is far weaker than the force of protoss Zealots, but Boxer runs his zerglings all over the place to prevent the zealots from doing much. Then Boxer splits his zerglings into two groups and runs them different directions. Furthermore, he BURROWS his zerglings when zealots reach them. The effect is that the Protoss player is forced to split his zealots in two groups also. If he does not, then one of the groups of zerglings could attack something uncontested. Look how much Boxer ruins the Protoss player's incompressible clicks with this, and how far behind the Protoss player gets.
Savior's Crazy Zerg Harass
This is quite a game to watch. Zerg (played by Savior) is always behind in army power during this game. There are many times when the Protoss's army (played by Stork) is ridiculously more powerful than the Zerg's, but Zerg uses extreme harassment tactics to stay in the game. By sending a tiny number of hydras to the Protoss base, he forces the Protoss army to come back home to stop it. This kind of thing happens over and over, at different parts of the map and in different ways, but the concept is always the same. The relatively weak zerg units put themselves into positions that the Protoss MUST respond do, so the Protoss player is never able mount a devastating attack.