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."
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