From a Cast Perspective, it works...poorly. I've seen CDS do some fairly wacky things in my time, like sending a Cast Member to position and then sending another Cast Member to send the first Cast Member to break 2 minutes later.
I don't know about
Disneyland, but at WDW, CDS, for the most part, works as designed, and is not nearly as loathed as glendalais' post made it seem.
The reason it may send someone to a position and send someone else to break them 2 minutes later is because the heart of CDS is what's known as the Activity Queue. The queue is composed of a lot of different pieces.
First are the positions. Every position has times that it will start ("pick up"), and end ("drop") every day. Some positions, like the control panel of a roller coaster, will pickup before the park opens, and drop after the park ends. Some others, like maybe a stroller position, might come up mid-day, and drop in the early evening. Each of these position pickup and drop times becomes an entry in the activity queue.
Next, add in something called a rotational bump. All the positions follow an order, and the rotational bump moves people to the next position, down the line. For example, the rotation order might be Greeter > Grouper > Panel > Unload. Whoever initiates the rotation will go to Greeter, then the person currently at greeter moves to grouper, and so on. At the end of the rotation bump, whoever is in the last position (in our example here, "Unload") will go back to the CDS computer for a new assignment. Rotation bumps are typically spread out through the day at an established interval. (45 minutes is common). These are added as entries to our activity queue.
Lastly, we have cast members. In a typical 8 hour day in attractions*, a cast member will have 2 15-minute breaks, and 1 30 minute lunch, in a 15-30-15 pattern. (*note: other lines of business may use different patterns ... i believe merchandise uses a 30-30 pattern, for example). The cast member will also have a bump out (translation: time to go), which typically comes about 5 minutes before their scheduled end of shift. All these become entries in our activity queue.
So now lets look at why CDS might do something like sending someone to a position for only 2 minutes before they get sent somewhere else
Here's our hypothetical activity queue
1. Rotation Bump - 13:25
2. Pickup Position Strollers - 13:30 - 18:00
3. Break Cast Member Donald Duck - 13:35 - 13:50
Donald is in our unload position. The rotation bump comes through, and Donald, being at the end, goes back to CDS for a new assignment. The next thing in our queue is "Pickup Strollers", so Donald goes to the stroller position. Right after that, Mickey is starting his shift, and he signs in to get an assignment, and it will give him "Donald is due for a break, please take over the position Strollers, and have Donald go to break until 13:50"
Thus, Donald only spends a few minutes at strollers before he goes on break. CDS doesn't think, it just goes by whatever the order is that's in the queue, mostly*.
(*This is a pretty basic example ... there's a little more to it, but this post is already long enough, and I'm just trying to give a basic overview, and to add enough information to explain a previous post. I'm an attractions coordinator, and use/manipulate CDS on a daily basis, believe me, I know it has it's limitations, but it also works well enough to get the job done, and in a lot of cases it works well enough that it can be a completely self-sufficient system)
Not all areas use CDS ... for example, I know Entertainment doesn't use it, and, as of the last time I picked up a shift in resorts, they hadn't started using it yet either.
