Sorcery is not actually a Power. Rather, there are two quite distinct Partial Powers, one each for Logrus and Pattern, which are strange enough that they get their own special little listing.
Just like in the DRPG, Sorcerous spells are long, complex affairs (they take at least the better part of an hour to cast), that require gesturing and speaking in strange languages. But all you can do with a spell is enact an effect that you could already create with Pattern or Logrus.
Let's take an example. Suppose you have Pattern Defense and Sorcery. If you want to raise a Pattern Defense, you can do it two ways: The conventional one (concentrate on the Pattern) or by casting a spell. The spell will take longer.
What's the point, then? Well, it turns out that what you get in exchange for all of this time is much better fine control. Looking over the description of Pattern Defense, you might notice that it does a couple of related things. It can make you difficult to move through Shadow, for example, and it can also make you difficult to Shapeshift. In fact, it does both, at once.
But let's suppose that you aren't worried about being Shapeshifted. All you're worried about is getting thrown around by a Shadowstorm. With a Pattern Defense spell, you can focus your effort, so that all of the energy that previously going into making you hard to Shapeshift is now focussed on making you hard to throw about through Shadow. So you get more efficiency out of your Pattern skill and Psyche. (The metaphysics of it are actually a bit more complex than that, but this is the gist).
The other thing that you might do is "save up effort." Suppose that you're going to be assaulted by a big Shadowstorm, but that it will pass over you in just a few minutes. You might put out a lot of effort, taking hours and hours, to create the spell (during which time you aren't defended), and then have it all released over the course of five minutes once you complete the spell. In this way, you can (sort of) trade Endurance for power (albeit not too efficiently).
You're probably still not impressed, huh? But that's just the basic version of Sorcery. The advanced version lets you hang spells on the Pattern or Logrus. That way, suppose you know that you'll be going into an area with tons of Shadow Storms... Tomorrow. So for all of today, you make a whole bunch of spells like the ones we've been describing above, and hang them on the Pattern (or, rather, your personal image of the Pattern). Then you get a full night's sleep and a big breakfast. Then, whenever you see a Shadow storm, you just speak a few words that release your spell, and you get the benefit of all of yesterday's effort... today.
Here's some bad news: Because the disciplines of Sorcery are so esoteric, Sorcery skill doesn't add to the rest of your Pattern or Logrus skill. That is, if you have all of the Pattern powers plus Sorcery, and Joe has all of the Pattern powers and no Sorcery (and Joe's psyche is the same as yours), you don't Hellride any faster than Joe, and your Pattern lens is no more capable of weaselling its way into warded areas than his.
Here's what you can do with Sorcery and Pattern
And here's what you can do with Logrus Sorcery.
Basic Sorcery costs 10 points. Advanced costs an additional 5. You can hang up to 10 spells on the Pattern (less if you're putting a whole lot of energy into each one), and 20 on the Logrus, but stuff on the Logrus gets stale faster. (Spells on the Pattern get stale, too. If your spell's on the Pattern, it's probably useable for a month or so. On the Logrus, one or two weeks).
Pattern and Logrus sorcery, despite their similarities, are different powers. If you have both Pattern and Logrus, and wish to get both kinds of Sorcery, you must pay the costs once for each Power. Sorcery is intentionally weaker than the DRPG book's version, and this weakness will be enforced in play!